"Trei-Asar" - meaning "Twelve" in Aramaic, which was the language spoken in the time of CHAZAL, specifically, the Anshei K'nesset HaGedolah, the Men of the Great Assembly, who by their holy intuition and use of logical principles, established the contents of the "TANACH" (an acronym standing for Torah, Neviim, K'tuvim), the Hebrew Bible.
The "Twelve" refers to a group of Prophets who prophesied over a period greater than two hundred years. This period begins towards the end of the Kingdom of Israel (as opposed to the Kingdom of Yehudah) in Shomron (ca. 722 B.C.E.) that involved the Exile of the "Ten Tribes," until and including the time of "Shivat Zion," the "Return to Zion" and the rebuilding of the First Temple in Jerusalem (ca. 516 B.C.E.).
The order of these Prophets, according to CHAZAL, that takes into account mainly chronology, but also thematic issues, is Hoshea, Yoel, Amos, Ovadiah, Yonah, Michah, Nachum, Chavakuk, Tzefaniah, Chaggai, Zechariah, Malachi.
These twelve also "belonged" to a group called "Minor Prophets," called that only because the volume of their recorded prophecies was relatively small, compared to the other Books of the Bible, but qualitatively, their works have significance "l'dorot," for all generations.
As a group, they rebuked the Jewish People about their continuous idol-worship and, possibly worse, for their worship of HaShem with the proper outward trappings, but with none of the required inwardness, reducing their great religion to a mockery and a meaningless shell of ritual.
They also harshly criticized the People for their lack of social justice, whereby they trampled upon the rights of the underprivileged. And yet another great theme was their reliance on foreign nations for their salvation, rather than upon HaShem, Who had stood by them always and had saved them from Egypt, the Seven Nations, Amalek and on and on.
But they also had immortal words of comfort for their People, of Redemption and Salvation to come, if the People of Israel would only do "Teshuvah," return to HaShem with all their heart and all their soul.