1-2: Obadiah's book is a message from God concerning the judgement and destruction of Esau's descendants, the Edomites. God sovereignly rose up surrounding nations to destroy them.
3-9: The Edomites lived in Petra (the clefts of the rock) which was very hard to attack because of it's location. They took comfort in this and became prideful that no one could defeat them. However, their neighbors and allies would eventually rise up and slaughter them completely.
10-16: This is God's indictment of Edom for their violence against Jacob (Israel). He tells them not to celebrate their invasion of Jerusalem because judgement is coming. The Edomites were completely destroyed in the Roman siege of Jerusalem in 70 AD.
17-21: On the Day of the Lord justice will be executed and the victims of violence will rule their enemy's land.