AC3 is another name for something you will probably know as Dolby Digital. The AC3 standard is maintained by the ATSC and can be downloaded free of charge at the ATSC website.

AC3 streams can carry everything between a simple mono channel and 6 separate channels. The most popular stream type is usually called 5.1 surround. 5 channels are used for 5 different surround speakers and the 6th (low bandwidth) channel is used for LFE (low frequency effects).

The official document name of the standard is: 

A/52 - Digital Audio Compression Standard (AC-3)

The document contains all you need to implement your own AC3 decoder or to extract only the stream information from the header. But... please note that you may not implement and distribute an AC3 decoder without paying license fees to Dolby Laboratories Inc. They are the inventor of this compression method and hold several patents for this audio compression standard.