It's a relatively simple fighting game between two teams of space ships. A team wins the round when all ships of the other team are destroyed, and a certain number of victories more than the other player is needed to win the game. But there's a catch: as a team gets closer to winning, it loses ships, so you need to be better than the other team to succeed.
Human players take over one ship at a time from their respective teams. If a human player's ship is destroyed, that human player takes over an AI ship from their team randomly. This ensures that the human players are always in the game.
Downloads and screenshots can be found on its website: http://pacewar.nongnu.org