Gheebah or backbiting means speaking about a person in his absence and saying things that he would not like to have spread around or mentioned. Buhtan or slander means saying things about a Muslim that are not true, or in other words telling lies about him. Nameemah or malicious gossip means telling one person what another said in order to cause trouble between them. 
O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah ; indeed, Allah is Accepting of repentance and Merciful. Qur’an 49:12
The Prophet Muhammad (peace_be_upon_him) said: O community of people, who believed by their tongue, and belief did not enter their hearts, do not back-bite Muslims, and do not search for their faults, for if anyone searches for their faults, Allah will search for his fault, and if Allah searches for the fault of anyone, He disgraces him in the open for everyone to see, even if he hid it in the innermost part of his house. Sunan Abu Dawud: Book 41, Number 4862, Sahih Sunan Al Tirmidhi (1655), Ibin Hiban, Hasan by Al Albani
Narrated Sahl bin Sa’d: Allah’s Apostle said, “Whoever can guarantee (the chastity of) what is between his two jaw-bones and what is between his two legs (i.e. his tongue and his private parts), I guarantee Paradise for him.” Sahih Al Bukhari Volume 8: 481
The Prophet Muhammad (peace_be_upon_him) described: “When I was taken up to heaven I passed by people who had nails of copper and were scratching their faces and their breasts. I said: Who are these people, Gabriel? He replied: They are those who were given to back biting and who aspersed people's honour”. Sunan Abu Dawud 4860.
The scholars have stated that gheebah is permitted in certain situations:
1. Complaining. It is permissible for the one who has been wronged to complain to the ruler or judge and others who have the authority or ability to settle the score with the one who wronged him.
2. Seeking help to change evil and bring the sinner back to the right path, so he may say to the one who he hopes is able to do something: “So and so is doing such and such; tell him not to do it.”
3. Seeking advice or a fatwa (religious ruling), by saying to the mufti (scholar), “So and so/my father/my brother has wronged me by doing such and such, does he have the right to do that? How can I solve this problem and ward off his harm from me?”
4. Warning the Muslims of someone’s evil, such as highlighting the weakness of some reporters or witnesses or authors. That also includes seeing someone buying faulty goods, or someone keeping company with one who is a thief or adulterer, or giving a female relative of his to such a man in marriage, and the like. You should tell them about that by way of sincere advice, not with the aim of causing harm and spreading mischief.
5. If a person openly commits evil or follows bid’ah (innovation), such as drinking alcohol and seizing people’s wealth unlawfully, it is permissible to speak of what he is doing openly, but it is not permissible to speak against him any other way, unless it is for another reason.
6. For identification, if someone is known by a nickname such as the dim-sighted one, or the blind man or the one-eyed or the lame one, it is permissible to identify him as such, but it is haraam (impermissible) to mention that by way of belittling him, and if it is possible to identify him in some other way, that is better. 
Whoever wrongs a person by slandering him, backbiting about him or insulting him, then repents, Allah will accept his repentance, but if the one who was wronged finds out about that, he has the right to settle the score. But if he slandered him or backbit about him and the person did not hear of that, then there are two views according to the scholars, both of which were narrated from Ahmad, the more correct of which is that he should not tell him that he spoke against him in his absence. It was said that he should rather speak well of him in his absence just as he spoke badly of him in his absence, as al-Hasan al-Basri said: the expiation for gheebah is to pray for forgiveness for the person about whom you backbite. Majmoo’ al-Fataawa Majmoo’ al-Fataawa, 3/291. 
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