Imams (a) of Shi'a are twelve men from the progeny of the Prophet (s) who are, according to Shi'a teachings, successor of the Prophet (s) and the guardians and leaders of the society after the Prophet (s). The first Imam (a) is Imam 'Ali (a) and the other Imams (a) are his sons and grandsons and lady Fatima (s).
These Imams (a) are appointed by God and have divine knowledge, infallibility and the right of intercession [for people] and one can become near God through making Tawassul to them. Imams (a) have the political leadership of the society as well as their scientific authority. Verses of the Qur'an without mentioning their names have discussed the issue of imamate; such as in verse of Uli al-Amr, Tathir, al-Wilaya, al-Ikmal, al-Tabligh and al-Sadiqin.
In some narrations from the Prophet (s), the qualities, the names and the number of Imams (a) have been mentioned; such as Hadith al-Thaqalayn, Hadith of Manzila, Hadith of Safina, Hadith of Yawm al-Dar, Hadith of Madinat al-'Ilm, Hadith of al-Tayr al-Mashwiyy, Hadith al-Rayat, Hadith al-Kisa', Hadith of Jabir and Hadith of the Twleve Caliphs. According to these narrations, all of Imams (a) are from Quraysh and they are the Ahl al-Bayt (a) (household) of the Prophet (s) and the last Imam (a) is the Promised Mahdi (aj).
Also, there are many hadiths from the Prophet (s) in hadith references about the imamate of Imam Ali (a) who is the first Imam (a). Also, there are hadiths from the Prophet (s) and Imam Ali (a) which have explicitly mentioned the imamate of the second Imam (a). Afterwards, every Imam (a) explicitly has introduced the Imam (a) after himself. According to these explicit mentioning, Imams (a) and the caliphs after the Prophet (s) are 12.
Source: http://www.wikishia.net/
