In religious traditions, a prophet is a messenger from God. The Jewish and Christian Bibles contain the teachings of many prophets. Muhammed is, according to Islam, the last of the prophets sent by God.
Muslims believe that the Qur'an (the sacred book of Islam) was revealed to the Prophet Muhammed. Since Muhammed is the last of the Prophets, the Qur'an must be God's final word.
When Muslims say the name of Muhammed, they often add the words "peace be upon him" (pbuh). This is a reminder that they have great respect for Muhammed as a human being and messenger of Allah.