Sefer Ovadyah (Obadiah) is the book of the vision of Ovadyah, a navi (prophet), concerning Edom, the nation of Esav (Esau), Yaakov's brother, whose land lies southeast of Eretz Yisrael (the Land of Israel). It is one of the Trei Asar (the Twelve), the twelve short books of the Neviim Acharonim (the Later Prophets), which the Gemara (Bava Batra 14b) says are written together as one sefer. Sefer Ovadyah has a single perek of twenty-one pesukim.
Hashem sends word among the nations to rise up against Edom. Edom lives high among the rocks and is sure that no one can bring it down, but Hashem will bring it down even from among the stars. Its wise men and its warriors will be lost, and its own allies will turn against it.
The reason the navi gives is the chamas (violence) Edom did to its brother Yaakov. When strangers came through the gates of Yerushalayim (Jerusalem) and cast lots over it, Edom stood by and acted like one of them. It gloated over the ruin of Bnei Yehudah (the people of Judah), took their wealth and handed over their survivors.
The day of Hashem is near for all the nations, and what Edom did will be done to it. Beit Yaakov (the house of Jacob) will be a fire and Beit Esav straw. The exiles of Yisrael will come back and possess the land, from the Negev to the Gilad. Deliverers will go up on Har Tziyon (Mount Zion) to judge Har Esav, and the kingdom will be Hashem's.
Sefer Ovadyah is read as the haftarah of Parshat Vayishlach.