Zulm (ظلم) means oppression. It could be oppression against Allah or oppression against humans, animals, etc. A Person who commits such oppression is a Zaalim (Oppressor) and the victim is called as Mazloom (Oppressed).
ظُلْم ( اسم ) : جَوْر
- complete power and authority
- great flood:heavy rush of water:violent rainfall
- tyranny: dictatorship: oppression
- the state of being highhanded
- injustice, unfairness
- lack of fairness
- the act or instance of oppressing; the state of being oppressed, or harsh and cruel treatment
- an extreme or shocking violation of others' right or fierce anger
- persecuting or being persecuted
- a political system in which ordinary people have no power and are completely controlled by the government.
- trespass or infringement .
- a voluntary wrongful act against the person or property of another, esp. unlawful entry to a person's land or property .
- noun gerund of verb ( to trespass )
- the cruel and arbitrary use of authority or a tyrannical act .
- the state of being unfair
- failing to comply with (an oath, treaty, law etc)
- what is morally wrong; a wrong action; injustice; unjust action or treatment
There are many aayaath about Zulm in the Qur’an. Some of them are:
1. And [recall] when Moses said to his people, "O my people, indeed you have wronged yourselves by your taking of the calf [for worship]. So repent to your Creator and kill yourselves. That is best for [all of] you in the sight of your Creator." Then He accepted your repentance; indeed, He is the Accepting of repentance, the Merciful.Surah Al Baqarah 2:54
2 .And We shaded you with clouds and sent down to you manna and quails, [saying], "Eat from the good things with which We have provided you." And they wronged Us not - but they were [only] wronging themselves. Surah Al Baqarah 2:57
3. The example of what they spend in this worldly life is like that of a wind containing frost which strikes the harvest of a people who have wronged themselves and destroys it. And Allah has not wronged them, but they wrong themselves. Surah Al e Imran 3:117
1. Jabir bin 'Abdullah (R) narrated that the Messenger of Allah (ﷺ) said: “Beware of oppression, for oppression will turn into excessive darkness on the Day of Resurrection and beware of niggardliness, for niggardliness destroyed your predecessors.” Sahih Muslim 1526.
2. 'Aishah (R) reported: Messenger of Allah (ﷺ) said, "Whoever usurps unlawfully even a hand span of land a collar measuring seven times (this) land will be placed around his neck on the Day of Resurrection". Sahih Al-Bukhari Vol.4: 3195
3. Allah's Messenger (ﷺ) said, "Help your brother, whether he is an oppressor or he is an oppressed one. People asked, "O Allah's Messenger (ﷺ)! It is all right to help him if he is oppressed, but how should we help him if he is an oppressor?" The Prophet (ﷺ) said, "By preventing him from oppressing others."Sahih al-Bukhari 2444
4. Abu Dharr (R) quoted the Prophet (ﷺ) saying among what he narrated from Allah, the Most High that He has said, “O MY slaves, I have made oppression unlawful for myself and I have made it unlawful among you, so do not oppress one another.” Sahih Muslim Book 16, Hadith 1537
5. Narrated Abu Sa'id al-Khudri (R):The Prophet (ﷺ) said: The best jihad in the path of Allah is (to speak) a word of justice to an oppressive ruler. Sunan Abi Dawud 4344.
6. Allah's Messenger (ﷺ) said, "Whoever has oppressed another person concerning his reputation or anything else, he should beg him to forgive him before the Day of Resurrection when there will be no money (to compensate for wrong deeds), but if he has good deeds, those good deeds will be taken from him according to his oppression which he has done, and if he has no good deeds, the sins of the oppressed person will be loaded on him." Sahih al-Bukhari 2449
7. Abu Hurairah (R) narrated that the Messenger of Allah ﷺsaid: "Three supplications are accepted, there is no doubt in them (about them being accepted): The supplication of the oppressed, the supplication of the traveler, and the supplication of his father against his son."Jami` at-Tirmidhi 1905
8. Abdullah b. Sarjis (R) reported that when Allah's Messenger (ﷺ) set forth on a journey, he sought refuge (with Allah) from the hardships of the travelling, and finding of evil changes on return, and disgrace after honour, and the curse of the oppressed and a gloomy sad scene in family and property.Sahih Muslim 1343 a
9. Abu Huraira(R) reported Allah's Messenger (ﷺ) as saying: When two persons indulge in hurling (abuses) upon one another, it would be the first one who would be the sinner so long as the oppressed does not transgress the limits.Sahih Muslim 2587
10. Allah's Messenger (ﷺ) said, "A Muslim is a brother of another Muslim. So he should neither oppress him nor hand him over to an oppressor. And whoever fulfilled the needs of his brother, Allah will fulfill his needs."Sahih al-Bukhari 6951
11. Narrated Ali ibn AbuTalib: The Prophet (ﷺ) said: If only one day of this time (world) remained, Allah would raise up a man from my family who would fill this earth with justice as it has been filled with oppression. Sunan Abi Dawud 4283
12. Abu Bakra reported that the Prophet, ﷺ, said, "There is no wrong action more likely to bring punishment in this world in addition to what is stored up in the Next World than oppression and severing ties of kinship." Al-Adab Al-Mufrad 29
It was narrated that ‘Abd-Allaah ibn Mas’ood (R) said: When the words “It is those who believe (in the Oneness of Allaah and worship none but Him Alone) and confuse not their Belief with Zulm (wrong)…” Surah al-‘An’aam 6:82 were revealed, the companions of the Messenger of Allaah ﷺwere distressed by that and said: Who among us has not wronged himself? The Messenger of Allaah ﷺsaid: “It is not as you think; rather it is shirk. Have you not heard what Luqmaan said to his son when exhorting him: ‘O my son! Join not in worship others with Allaah. Verily, joining others in worship with Allaah is a great Zulm (wrong) indeed’ Surah Luqmaan 31:13?” Sahih al-Bukhaari (3246) and Muslim (124). 
The position of the oppressed in relation to the oppressor: Allaah Says (what means): “And those who, when tyranny strikes them, they defend themselves. And the retribution for an evil act is an evil one like it, but whoever pardons and makes reconciliation - his reward is [due] from Allah. Indeed, He does not like wrongdoers. And whoever avenges himself after having been wronged - those have not upon them any cause [for blame]. The cause is only against the ones who wrong the people and tyrannise upon the earth without right. Those will have a painful punishment. And whoever is patient and forgives - indeed, that is of the matters [requiring] determination [i.e. on the part of those seeking the reward of Allah].” Quran 42: 39-43
Theses aayaath include two matters:
First: Taking revenge from the oppressor.
Second: Forgiveness on the part of the oppressed.
Ibraaheem may Allah have mercy upon him said: “The believers hated anyone to humiliate them, but whenever they had the chance to take revenge, they would forgive instead.”
Allaah praises the believers in the above mentioned aayaath for having the strength and power to avenge in a just manner, for restoring their rights, and for not allowing aggressors to take advantage of them. They are by no means incapable, weak or humiliated, and they are quite capable of exacting revenge, yet they forgive and forget.
There are many Quranic aayaath and prophetic narrations addressing this issue and encouraging the oppressed to forgive the oppressor when he is in a position to exact revenge, as this will boost his record of rewards on the Day of Resurrection. On the other hand, if the oppressed insists on revenge then this is his right and it is perfectly permissible to do so, but the revenge must be strictly limited to the extent that he was wronged. If the wronged person is one of two wives, it is permissible for the husband to enable the wronged one to gain revenge, as the Prophet ﷺpermitted one of his wives to get back at another one who had badmouthed her.
Among the things that the Prophet ﷺsaid to Mu`âdh(R) when he dispatched him to govern over Yemen was: “And be wary of the supplication of the oppressed, for between it and Allah there is no barrier.” Sahîh al-Bukhârî (1496, 2448) and Sahîh Muslim (19)
This is understood to mean that the supplication of the oppressed against his oppressor is answered.
It is clear from the context that this is a strong warning against oppressing others. The Prophet ﷺwas warning Mu`âdh (R) to be wary of wronging the people under his authority.
It should not be regarded as an encouragement to invoke supplications against other people every time they do something wrong to you. It is better to pray for their forgiveness and to bear their misdeeds with patience.
At the same time, supplication is the refuge of the truly oppressed and downtrodden. When oppression and weakness strips a person of all practical hope and leaves that person with no one to give support against those who oppress him, that person raises his or her hands to the sky and sets forth his or her complaint before the Lord, the Almighty. Allah strengthens that oppressed person and brings relief, and He lets His wrath fall upon those who oppress.
We see that Noah (peace be upon him) supplicated against his people when they oppressed him, spurned him, and rejected his call.
Also, when Moses (peace be upon him) beseeched Allah in supplication against Pharaoh after he had transgressed, oppressed, and tyrannized Moses's people, Allah answered his supplication. The oppressor’s lot is disgrace in this world and a painful chastisement in the Hereafter.
`Umar said, when he appointed Hunayy to manage the affairs of an area: “Restrain your hands from wronging the Muslims and be wary of the supplication of the oppressed, for indeed the supplication of the oppressed is answered.” Sahîh al-Bukhârî (3059)
The one who oppresses others should fear Allah and beware of transgression, because it will be counted as multiples of transgressions on the Day of Resurrection: one for him disobeying Allah, another for him oppressing his fellow Muslim, and a third for initiating and opening the gate of transgression and oppression for others to imitate. The wronged person could even eventually die as a result of the oppression, and the transgressor would bear the consequences of this.
Aslam (R) narrated: “`Umar Ibn Al-Khattaab appointed a freed slave of his, called Hunayy, as manager of the Himaa (i.e. a pasture devoted for grazing the animals of the Zakaah or other specified animals, such as those used for fighting etc.). He said to him: 'O Hunayy! Do not oppress the Muslims and (be sure to) ward off their curse (i.e. invocations against you), for the invocation of the oppressed is responded to (by Allah); allow the shepherd having a few camels and those having a few sheep (to graze their animals in that land), and take care not to allow the livestock of 'Abdur-Rahmaan Ibn 'Awf and the livestock of ('Uthmaan) Ibn 'Affaan (to graze their animals in your land), for if their livestock should perish, then they still have their farms and gardens, while for those who own only a few camels or sheep, if their livestock should perish, they would bring their dependents to me and appeal for help saying: 'O Chief of the Believers! O Chief of the Believers!' Would I then neglect them? (No, of course not).
So I find it easier to let them have water and grass than to give them gold and silver (from the Muslim treasury). By Allah! These people think that I have been unjust to them. This is their land, and during the pre-lslamic period, they fought for it but then they embraced Islam (willingly) while it was in their possession. I swear By Him in whose Hand my life is! Were it not for the animals (in my custody) which I give to be ridden (for striving in Allah's Cause), I would not have turned even a span of their land into a Himaa.'” Sahih Al-Bukhari 3059
This was said regarding a matter that was not personal and he was merely being careful not to utilize the Muslim property in an incorrect manner, and yet he feared being unjust in any way. Therefore, those who transgress other people’s rights should fear Allah and fear the oppressed supplicating against them, as this is a very dangerous matter.
The transgressor must return the rights of people to them or their equivalent:
How can the transgressor free himself from his transgression? Imam As-Safaareeni may Allah have mercy upon him said: “The rights of people are either of a type that can be compensated for by an equivalent, like wealth or ruined goods, or by a type that cannot be compensated for.” This means that if a person has unjustly taken wealth, then he should return it; if he ruined something, then he should replace it; if he took something and used it, then he should return it and compensate the owner for the usage. These steps should all be accompanied with sincere repentance to Allah.
That is in the case of it being something that can be compensated for, but what should the transgressor do in the case that it is of the other type, which cannot be compensated for? Slandering a person or backbiting him are not things that can be compensated for, and in such a case one should first of all regret what he did and repent, then he should supplicate for the forgiveness of the one whom he slandered or backbit. If the person falsely accused another of adultery or fornication, then he should go back and inform those in front of whom he made the accusation that he was lying.
Furthermore, he should seek the pardon of the wronged person unless he knows that by informing him things would only get worse, in which case he should refrain from telling him and simply supplicate earnestly to Allah for the wronged person's forgiveness; he should also praise the wronged person in front of those same people that he had defamed or slandered him. 
After knowing the positions of the transgressor and the transgressed against, we need to know the stance that people should adopt towards each of them.
Anas (R) reported: Allah's Messenger (ﷺ) said, "Help your brother, whether he is an oppressor or he is an oppressed one. People asked, "O Allah's Messenger (ﷺ)! It is all right to help him if he is oppressed, but how should we help him if he is an oppressor?" The Prophet (ﷺ) said, "By preventing him from oppressing others."Sahih al-Bukhari 2444
Al-Baraa’ ﷺreported: “The Messenger of Allah ﷺcommanded us with seven things (one of which is): “Support the transgressed.”
Ibn `Umar (R), reported that the Messenger of Allah said: “A Muslim is a brother of another Muslim. (So) he should not oppress him, nor should he forsake him (i.e. when he is being oppressed).” Sahih Al-Bukhari 6951
Therefore, one must support the oppressed and the one who is being transgressed against; the one who refrains from supporting an oppressed person, whilst being able to, is sinful.
Abu Bakr ‘(R) said: “O people! You recite this ayath (which means): “O you who believe! Upon you is [responsibility for] yourselves. Those who have gone astray will not harm you when you have been guided...”Surah Al Maidah 5:105 (but misinterpret it) for I have heard the Messenger of Allah ﷺ saying: 'When people see an oppressor but do not prevent him from (doing evil), it is likely that Allah will punish them all.'” Abu Dawud 4338 and At-Tirmithi 2168
One should prevent the oppressor physically if possible, and this responsibility is greater with respect to the rulers and those in authority. If one cannot stop the oppressor physically, then he should do it verbally. 
Helping an oppressor is from the gravest of disasters; people should not imitate an oppressor and must prevent him and make him realize the wrong that he is doing; if people assist the oppressor then they will not get to drink from the fountain of the Prophet ﷺas He ﷺhimself informed us of this being the case.
Some people support a transgressor because he is a relative or a fellow tribe member, but the Prophet ﷺwarned against such oppressive behavior.
All that has been mentioned thus far does not suggest that one should be easily deceived by someone who claims to have been oppressed, for it is not necessarily true that every person who claims to have been oppressed has actually been; the following is a case in point:
During the era of Abu Bakr (R) a man whose hand and foot had been amputated came to Al-Madeenah from Yemen and went to Abu Bakr (R) complaining that the governor there had wronged him. A few days later, a necklace of Asmaa’ Bint ‘Umays (R) the wife of Abu Bakr (R) was stolen and so people began searching for it.
The Yemeni amputee assisted in the search and supplicated: “O Allah! Punish the one who stole from this righteous household.” Finally the necklace was found in a jewellery store. After investigating the matter, the jeweller claimed that the man from Yemen was the one who brought it to him, and when he was questioned he confessed to his crime. Therefore, (R) commanded that his other hand be amputated and said: “I swear by Allah! His supplication against himself was worse than his theft.” Although this man claimed that he had been oppressed, it was subsequently discovered that he had not been, and that his hand and foot were in fact rightfully amputated.