Quoting the official YouTube blog:
Making video easily accessible is something we're working hard to address at YouTube. One of the first steps we took was the development of a caption feature in 2008. In November of last year we released auto-captioning for a small, select group of partners. Auto-captioning combines some of the speech-to-text algorithms found in Google's Voice Search to automatically generate video captions when requested by a viewer. The video owner can also download the auto-generated captions, improve them, and upload the new version. Viewers can even choose an option to translate those captions into any one of 50 different languages -- all in just a couple of clicks.