In the Portal settings, I can choose between "no subtitles", "closed captions" and "subtitles". What's the difference between the second two?
Subtitles generally display only what is spoken by a character. They are intended for players who are able to hear, but prefer to have the dialogue of the game displayed.
Closed captions provide a way for those who may not be able to hear the game sounds at all to fully enjoy the game. They include dialogue, sound cues (such as "machinery starting up"), and they don't contain jokes by the developers which could lead to misunderstanding by a player who is unable to compare the actual audio to the subtitles (for example, in the subtitles, the line in GLaDOS's song "when I delete you maybe I'll stop feeling so bad" is marked as "REDACTED", because she doesn't want to admit to liking the player. In the closed captions, this line is presented correctly so that a non-hearing player can gain the full understanding of the song)
Closed captions refers to subtitles for every sound (even falling objects and onomatopoeias), while subtitles refers to spoken words only.