A player is a participant in a StarCraft game. Players own and control units in a StarCraft map. Vision and triggers also operate on a per-player basis. A map cannot have more than eight players (excluding Neutral).
Virtually every StarCraft map features a human player. Human players are the actual people that play maps.
Computer players' actions are controlled by AI scripts and triggers. The mapmaker has control over computers' actions.
A computer player need not be an actual participant in the game. A mapmaker might use a computer as a trigger or death counter owner, to ensure that certain important triggers run regardless of the number of human players. Computers could also be used to give a human player vision of one or more specific locations.
Rescuable players are a special kind of computer players. When another player's units approach those of a rescuable player, the other player takes ownership of the rescuable units.
Neutral, also known as Player 12, is a "dummy" player. All Mineral Fields and Vespene Geysers on a map belong to Neutral. If, during an online match, a human player leaves the game early, their units will be given to Neutral. Unlike normal human and computer players, Neutral cannot run triggers. (Triggers can be given to Neutral, but they will not execute unless other players also own them.)
Players 9, 10, and 11 are extended players. These players can only be made use of in third-party map editors.
Extended players do not have vision of any units -- not even their own. This means that, with few exceptions, extended players' units cannot attack other units. Like Neutral, extended players cannot run triggers; this means that they cannot gain shared vision of other players' units.