DVD Audio is, as the name would imply, a scheme to put purely audio data on DVD media. In basic terms it means more length with more better sound, but it's actually much more then that when you look at it in technical terms.
Though the standards regarding the physical state of DVDs has been around since about 1996, the data storage specification for puting pure Audio on those discs didn't come around until March 1999 when the DVD Forum released the final version 1.0 of the DVD-Audio specification. While this seems like a big spread, let's not forget that the CD was around in the early 1970's but the public wasn't able to get it until the mid 1980's, ten years later; by that standard, the DVD Audio design was rushed through.
As said, physically a DVD Audio package is just plain vanilla Digital Versatile Disc with the data encoded in a special format; You may have even seen how -- many DVD Video's will have two directories on them in their UDF structure, a TS_VIDEO and TS_AUDIO, though it's not clear if DVD Audio is required to inhabit the TS_AUDIO directory.
In basic design a DVD Audio package works a lot like an CD, except with up to 15 times the capacity; they both can use PCM and are arranged in tracks that are read linearly -- but DVDA exploits it's extra capcity by increasing the sample rate from 44.1kHz as CDs have to a maximum of 192kHz. The dynamic range and frequency response goes up too, from 96dB to 144dB and from 20kHz to 96kHz, respectively. Sample size is variable on DVD-A, too, and can be 12, 16,20 or 24 bits wide instead of just 16 bits for Red Book CD or 20 bits for Duplication Masters.
And yet, there is still more in the audio coding front -- if you want to increase play time from it's standard PCM runtime, the DVDA spec also includes a option for using Meridian Lossless Packing, a lossless compression scheme that is supposed to achieve a uniform 2:1 compression to push audio capacity to 2 hours for 6-channel 24-bit, 96kHz audio (or theoretically 6 hours for two channel audio of the same format).
What else in the goody bag? The option for individual tracks (including sub tracks of the same master track) to have different encoding methodologies -- for example, if you had 6 channel audio, you could have the two forward channels encoded at 24-bit/96kHz and the 4 rear channels could be at a lower 16-bit/48kHz. In addition to all that, a DVD Audio disc can also contain a limited amount of arbitrary data, which can be used to display text, such as lyrics or notes, or stills such as a cover art. Up to 16 graphic stills can be associated with each track as well as on-screen displays that can be used for disc navigation. Optionally, a DVD Audio disc can also include "audio" tracks recorded in the DVD Video specification (Dolby Digital and DTS) for compatibility with DVD-Video players.
Of course, you need a DVD player capability of decoding DVD audio -- for computers this is no problem, just download software. But in set-top players, this may be an issue since very few players, even those made after the specification was official, support DVD audio, and those that do are very expensive. So you can either hope your player is compatible, buy a new DVD Video player that is also DVD Audio ready or get a stand-alone DVD audio player.
DVD Audio is seen by most people as the arch-nemesis of Sony's SACD. Few people care about SACD, or DVD Audio for that matter, so not many people are paying attention. Except you, you geek.
(2012-05-22) DTal says: I am not aware of any computer programs to play DVD-Audio discs. A very, very few claim to be able to rip them to wavs - I did this successfully with a fairly underground piece of software (it didn't have a website; it was an attachment on a forum post). This was with an unencrypted disc, too - DRM applies to DVD-A just as much as DVD video, with the additional caveat that not many people are working on breaking it.