The four Old Testament figures considered to be major prophets because their books are longer are Isaiah, Jeremiah, Ezekiel and Daniel. The 12 minor prophets include Joel, Amos, Micah, Habakkuk and Zechariah.
Isaiah is called the evangelical prophet because he mentions the word "salvation" 23 times while the other major prophets mention it just seven times. Isaiah wrote of the saving grace of the Messiah.
Jeremiah is called the "prophet of loneliness" or "the weeping prophet" because God commanded him not to get married. Jeremiah issued stern warnings about sin but also offered hope if the people would repent. He referred to Babylon 164 times, more than the rest of the Bible combined. He is also said to have written the book of Lamentations.
Ezekiel wrote of Israel's sins, which were so great God allowed the people's captivity in Babylon to correct them. Ezekiel prophesied about God's intent to establish a new theocracy for a chastened remnant of the people who repented sufficiently.
The book of Daniel speaks of the one true God who intends to come and deliver his people from their wickedness. God's plan is to establish the Messiah as ruler of the Earth.
The minor prophets aren't considered less important but are called minor because their books are shorter. Many of them complain of Israel's sins and look for a New Covenant to be established by the Messiah. Other minor Old Testament prophets include Hosea, Obadiah, Zephaniah, Haggai and Malachi, in addition to Nahum and Jonah.