Open captioning, which existed prior to closed captioning, meant printing the actual words onto the screen. This method is often used in films as an alternative to overdubbing with voice actors. Closed captioning is submitted via teletext in the vertical line blanking interval of televisions with appropriate encoders.
BBC's new iPlayer apparently has teething problems with subtitles but once they are sorted it will be the next step in subtitling:
About.com - a history of closed captioning
Subtitles, from Wikipedia