Build Battle is a competition between 2 or more players. Players are given a theme, either chosen randomly, choosing a theme, or voted. After the timer starts, players has to build something related to the theme. After the timer stops, the players look at others' builds, and voting using items in their hot bar. You cannot vote for yourself though. The best voted build wins.
Once the player gets into a game, there is a certain amount of time, usually 4-6 minutes, for the player to build whatever the theme is. The theme is either randomly chosen, a player who is a certain rank can choose the theme, or the theme is voted on. Players are in Creative Mode for this, and have access to the full Creative inventory. Special effects can be enabled through the settings. When building is over, players rate the builds. You cannot rate your own build. The player whose build received the most points wins.