A vote is placed on a player that you find suspicious and you would like to have executed. You vote by typing ##vote: PlayerName in thread. If you are no longer suspicious of a player, or you want to vote for someone else, you type ##unvote: PlayerName. The formatting is important, especially when moderators use automated votecount tools.
With the exception of some situations (e.g. some double voters), all votes are made public. When a majority of players have all voted on the same target, that target is executed and removed from the game. Analysis of votes cast after the execution is an important tool to helping track down scum.