There are three types of battlegrounds:
Siege rCTF - Hotjoin cCTF - Competitive
Siege consists of two teams, offense and defense, fighting against each other to either kill or defend the King of the map. Between each team lies a series of walls and doors that can be destroyed with new siege weaponry.
Capture The Flag
CTF is the basic CTF game play. Your team has a flag which you want to protect. You must go to the enemy team's base, grab their flag, and return it to your flag in your base to capture. If your flag is not at home you will have to return it before you're able to cap.
Read about Battlegrounds Wiki