A prophet can mean the same thing as a preacher. It can also refer to someone who has insight into the future or predicts future events, which is called a prophecy. Many books in the Old Testament were written by or about prophets and contained their messages for repentance or predictions for future events. Many of these prophetic predictions anticipated Jesus, though some had a more current focus, while a few looked at the end times.
Major Prophets are not those that are more important, but merely those who wrote longer books. (Compare to Minor Prophets.)
The Major Prophets are: Isaiah, Jeremiah, Ezekiel, and Daniel.
Four of these books are named after their respective prophet. The fifth, Lamentations, was also written by Jeremiah.