DVD-Audio is one of the standards controlled by DVD Forum, an authority behind various DVD standards. DVD-Audio is music disc, aimed to replace regular audio CD within next few years. DVD-Audio disc can contain 4 - 18 GBs of data, depending on how many layers and sides of the disc are used. Regular DVD-Audio disc contains various copy-protection methods and obviously the audio data itself -- audio can be stored in various formats, including uncompressed (L)PCM format (in frequencies from 44.1kHz upto 192kHz) with bitrates as high as 9.8Mbit/sec. Other formats allowed include Dolby Digital 5.1, MPEG-1 (stereo, audio layers II and III), MPEG-2 (multichannel), DTS and SDDS.
Another variety of DVD-Audio is called DVD-AudioV which adds a possibility to have regular DVD-Video compliant video on the disc as well.
DVD-Audio discs can't be played with regular DVD-Video players -- the player has to be so-called "Universal DVD" player in order to support DVD-Audio (obviously if your player has DVD-Audio logo, it supports these discs).