A prophet is one who speaks for God through divine revelation. Prophets in the Bible usually predict future events and the words of a prophet must always come to pass. If they do not, then he is a false prophet and must be put to death according to the book of Deuteronomy.
Prophets in the Old Testament also have the distinction of being largely ignored by the Jewish people who they are trying to reach.
A large portion of the Bible deals with prophets and prophecy, including two sections of the books of the Bible, the Major Prophets and the Minor Prophets. Note that the Minor Prophets is in reference to the length of their texts, not that they themselves were less of a prophet than the Major Prophets.
The Major Prophets:
The Minor Prophets:
There are other prophets in the Bible as well that don't have books named after them, such as Elijah and Elisha. Furthermore, while discussion of prophets is less pronounced in the New Testament of the Bible than it is in the Old, there are prophets in the New Testament as well, such as Agabus.
Prophets outside the Bible
Charismatic preachers, especially in the faith movement, will often refer to themselves as prophets.