AC 5.1 is a generic description of a multi-channel sound system. All of these surround sound types use 5.1 channel decoding to provide surround sound. The 5.1 channels are as follows:
1. Left Front
2. Right Front
3. Center Channel
4. Left Rear
5. Right Rear
6. LFE (Subwoofer)
AC3 is a compression technology term, which is used in place of Dolby Digital. It is essentially Dolby Digital.
Dolby Digital surround sound is compressed which allows more audio information to fit on a DVD disc.
DTS surround sound is an alternate AC 5.1 surround sound which DTS states is not as compressed as Dolby Digital.