There are also 6.1 versions of both DD and DTS. One is called ES, the other is called EX, and I can never remember which is which.
Anyways, there were two competing standards (DD and DTS); DD got chosen to be the DVD standard format but lots of people say DTS is technically superior. Other people say the DTS soundtracks are just recorded a bit hotter to make them sound better than the DD soundtrack.
All DVDs have a DD soundtrack, many also have DTS. I think we tend to pick the DTS soundtrack given a choice, but that might just to show that we know *how* to pick the DTS soundtrack
