An AI fulfils a similar role to a player, being there to control a team.
What most people would naturally call a team is called an “allyteam” in Recoil terminology.
Gaia is a special team that is always present, uncontrolled, and is always in its own allyteam.
Lua AI
A Lua AI generally has two components: a piece of game mechanics, and the AI instance itself which is just a handle to tell game mechanics which teams are legal to control.
Skirmish AI
A “skirmish” AI is hosted by one of the players and generally acts very similar to a player.
Spectators are players not controlling any team.
A team is generally controlled by a player or an AI.