The Da'wah literally means "call" or "Inviting people to Islam", and in this sense it refers to calling to the truth, preaching and propagation of the Qur'an and the Sunnah (i.e. the Religion of Islam - pure and free from any and all additions, deletions and alterations). It means adherence to the Path of Messenger of Allah (May Allah honor Him and grant Him peace), and the Faithful Believers, namely the Pious Forefathers (i.e. Pious Predecessors) of the Islamic Community of Believers and all those who follow in their footsteps in belief, actions and morals (May Allah Have Mercy on them).
The Qur'an is Allah’s (Glory be to Him) message for the whole humanity to come till the end of this world and it has laid great emphasis on Daw’ah. In fact a Muslim's life is supposed to be a "Da’wah" for mankind. Da'wah in Islam is actually an order from Allah in His Qur'an in a number of places, Qur’an. Surah Nahl 16:125, Surah Muhammad 47:38
The best person in the sight of Allah (Glory be to Him) is the one who do Da’wah i.e., inviting Mankind towards Allah (Glory be to Him). And who is better in speech than one who invites to Allah and does righteousness and says, "Indeed, I am of the Muslims." Qur'an.Surah Fusilat 41:33
Allah (Glory be to Him) has said: And whoever contends with and contradicts the Messenger after guidance has been clearly conveyed to him and chooses a path other than that of the Faithful Believers, We shall leave him in the Path he has chosen and land him in Hell, what an evil refuge! Qur’an.Surah Nisa 4:115
Shaikh-ul-Islam Ibn Taymiyyah(May Allah Have Mercy on Him) commented on this ayath: "All who contradict and oppose the Messenger after the right path has been clearly shown to them, have followed other than the Path of the Believers; and all who follow other than the Path of the Believers have contradicted and opposed The Messenger of Allah (May Allah honor Him and grant Him peace) after the right Path has been shown to them. If one thinks that he is following the Path of the Faithful Believers and is mistaken, he is in the same position as one who thinks he is following The Messenger of Allah (May Allah honor Him and grant Him peace) and is mistaken."
Da’wah is the work of all the messengers, inviting Mankind to worship one God ie, Allah (Glory be to Him)and to avoid worshiping all deities that exist beside Allah as mentioned in the Qur’an. Surah Nahl 16:36, Surah anbiya 21:25.
The best words that any human being can speak are words of guidance inviting people to the purpose of their creation; the worship of Allah (Glory be to Him). This being the case, the reward for giving da‘wah must be tremendous. Thus, it is no surprise to find that the Prophet Muhammad (May Allah honour Him and Grant Him peace)addressed the great reward for this righteous pursuit saying, “Whoever directs someone to do good will gain the same reward as the one who does good.” Sahih Al Muslim Vol 3: 4665
With regard to its obligation, Shaik Ibn Baaz (May Allah have Mercy on Him) said the following: “Da‘wah is an obligation on everyone who has the ability, from scholars to Muslim rulers until the message of Islam reaches every corner of the earth in the various languages of the people.
There are two levels of Da‘wah: The first is Fard ‘ayn(an individual obligatory duty) and the second is Fard kifaayah(a collective obligatory duty). It is Fard ‘ayn on you when no one in [your] country, region or tribe takes up the responsibility of enjoining good and forbidding evil, if you have knowledge. It becomes obligatory on you specifically to give da‘wah, to enlighten people to the rights of Allah (Glory be to Him), to command what is good and prohibit evil. However, if there are present those who give da‘wah teach people and guide them, then it would be sunnah and not obligatory for others who also have knowledge of the Sharee‘ah. Regarding the obligation of da‘wah in this age, Shaik Ibn Baaz (May Allah have Mercy on Him) also said: “At a time when there is a shortage of callers, when evil is prevalent and ignorance dominates, da‘wah becomes fard ‘ayn on everyone according to their ability.”