An Imam is considered a leader in Islam. He is the spiritual head of the Muslims and a leader of prayer in the mosque. An Imam must be very well versed in Arabic, in the dialect of his tribe. Shiites believed that Imams are infallible.
The word Imam literally means "one who is imitated". He is the person who leads the prayers. In the Muslim community the Imam occupies the highest place of honor. This honor is bestowed upon him because of his righteousness.
An Imam must have a great knowledge of the Qur'an, and must not receive any payment for leading the prayers. The blind and women are permitted to act as an Imam.
Muhammad acted as an Imam up to the time he became ill, and could no longer lead the prayers. He then appointed Abu Bahr to act has the Imam.
The Qur'an states that Abraham was made a leader (Imam) by God after he was found to be obedient to His commandments.
YUSUFALI: And remember that Abraham was tried by his Lord with certain commands, which he fulfilled: He said: "I will make thee an Imam to the Nations." He pleaded: "And also (Imams) from my offspring!" He answered: "But My Promise is not within the reach of evil-doers."
Surah 002.124