"Games [CC] is a dedicated group of captioners, translators, artists and programmers who modify exsisting games to add closed captioning to them. While most games (but not all) include subtitles for dialogue, many do not caption the sound effects from weapons or enemies. Adding closed captions helps hearing impaired and deaf players to experience the game as intended by being aware of the sound clues and information they may miss without the use of closed captions."
Closed Captions are the same as the UK Teletext subtitles service for television programmes. On-screen text relays what is being said (with different people having different coloured text) as well as descriptions of any other sounds and music. This can be switched on or off, and is a great boon for hearing and deaf people alike (especially when you have noisy relatives around!).