Bismillah
The Salafi Da'wah (da'wah literally means "call", and in this sense it refers to calling to the truth, preaching and propagation) is that 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 the Messenger, may the peace of Allah upon him, and the Faithful Believers, namely the Pious Forefathers (i.e. As-Salaf as-Saalih) of the Islamic Community of Believers and all those who follow in their footsteps in belief, actions and morals.
Allah 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! [Surah al-Nisaa' 4:115]
Shaikh-ul-Islam Ibn Taymiyyah commented on this verse: "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 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 and is mistaken."
Who are the Salaf and who are the Salafis?
The Pious Forefathers (i.e. As-Salaf as-Saalih) of the Islamic Community of Believers are the Companions of the Prophet, peace be upon him, their Followers (the Taabi'een and the Taabi Taabi'een (i.e. the first three generations of Muslims) and the Scholars of the Ahl-us-Sunnah wal-Jamaa'ah after them who followed their way in belief and deed. Among them are :
Imam Abu Hanifah (150 AH), Al-Awzai (157 AH), Ath-Thawri (161AH), Al Laith ibn Saad (175 AH), Imam Malik (179 AH), Abdullah ibn al-Mubarak (181 AH), Sufyan ibn Uyainah (198 AH), Imam Ash-Shafi'i (204 AH), Ishaq (238 AH), Imam Ahmed bin Hanbal (241 AH), Al-Bukhari (256 AH), Muslim (261 AH), Abu Dawood (275 AH) and others.
Ibn Taymiyyah (728 AH), and his students: Adh-Dhahabi (748 AH), Ibn al-Qayyim (751 AH), Ibn Katheer (774 AH) and others.
Muhammed bin 'Abd al-Wahhab (1206 AH) and many of his students.
And in our time: Abdul Aziz bin Baz, Muhammed bin Saalih Uthaimeen, Muhammed Naasir-ud-Deen al-Albaani.