In the Old Testament, a prophet was someone who brought the word of God to the people by the power of the Holy Spirit. “He has spoken through the prophets,” as we say in the Nicene Creed.
Sixteen books in the Old Testament (the “prophetic books”) are by or about these prophets. The four “major prophets” are Isaiah, Jeremiah, Ezekiel, and Daniel. The 12 “minor prophets” are Hosea, Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, and Malachi. Of course, Moses, the greatest Old Testament prophet, is traditionally regarded as the author of the first five books of the Bible. There are also many noteworthy prophets who did not contribute to the canon of the Bible, such as Samuel, Nathan, and Elisha. There were female prophets too: Miriam, Deborah, Huldah, Anna, and Philip’s four daughters.
Sometimes, in speaking for God, a prophet would foretell the future, and the legitimacy of his prophetic office was confirmed when his prophecies came true. Often his role was to convict the Israelites of their sin and to implore them to return to the Lord and remember the covenants He established with them. A prophet would also interpret current events in light of God’s plan for his people and warn them of His impending judgment (which usually came by way of the Israelites being conquered by their enemies). After the exile, when all the nations of Israel were scattered, the prophetic message was one of accepting the justice of God’s punishments and preparing for the coming Messiah.
A genuine Old Testament prophet was always directly called by God Himself, and he received the message he was to deliver by way of visions, dreams, and audible encounters. The prophets always spoke the “hard truths” that no one wanted to hear, and as a result they were often persecuted by their own people. The last and greatest of the prophets was John the Baptist, who was the immediate precursor of the Messiah and paved the way for Him with his message of repentance.
Jesus, of course, is the Priest, Prophet, and King par excellence, and in Baptism we are empowered to participate in that three-fold ministry of Christ. We are prophets today whenever we speak the truth with boldness, convict people of their sin, bring people to Christ, or share the teachings of His Church with others. Such actions often require sacrifice, just as they did in Old Testament times, but it is a calling that we all have been given and that we must undertake.
I hope that helps.