.ac3

Dolby Digital (AC-3) is Dolby's third generation audio coding algorithm. It is a perceptual coding algorithm developed to allow the use of lower data rates with a minimum of perceived degration of sound quality.

AC3 audio is used on pretty much every single retail DVD you will buy. To play back AC3 audio on your computer, you should install the AC3Filters. This will also play back AC3 audio in DivX or XviD files.