Dolby Labs and DTS are competitors, and each company masters digital movie sountracks (in Dolby's case, it's Dolby Digital). These are 5.1 formats for movie sountracks, but are not compatible with one another (i.e. your receiver/processor will need both onboard decoders to decode both the DD and DTS sountracks on a DVD).

DD EX soundtracks also contain a matrixed sixth channel for a speaker directly behind the listening position (the side surrounds are generally placed to the side and a little behind the listener). To decode EX soundtracks, you will need a receiver with EX capability (or an analogous 6.1 decoding chip like in the Outlaw 1050), plus 1 or 2 speakers for the back surround. All EX soundtracks are backwards-compatible with DD 5.1, so fear not if you've only bought 5 speakers and a sub.

DTS has a similar 6.1 system called DTS-ES. In a few rare instances (such as the Gladiator DVD), the surround back channel is actually encoded as a discrete channel, or DTS-ES Discrete. Other DTS-ES soundtracks use a matrix-encoded back surround channel much like DD EX. DTS-ES DVDs are also fully compatible with 5.1 systems.

DTS 96/24 is an encoding of music discs (which I *think* can be either DVDs or CDs, somebody correct me if I'm wrong) that contain a high-resolution recording (96kHz/24bit) which requires a DTS decoder to play. A receiver with a DTS decoder should have no problem with DTS 96/24 playback.

Neo:6 is a DSP mode on new receivers developed by DTS to derive surround sound from a 2-channel source, such as a CD or VHS tape. Dolby has its own patented DSP called ProLogic2 (which replaced the old ProLogic). Every receiver will have a pile of generic DSP modes as well, usually called things like Hall, Jazz and Cinema. I've heard mixed reviews of Neo:6 (ProLogic2 seems to be more popular), but DSPs are very much a personal taste thing anyway.

New receivers/processors decode all of these signals, unless the unit has no provision for a surround back speaker (via a powered channel or pre-out), in which case it will not decode the back surround channel. Personally, I don't think the surround back channel adds a whole lot if you're using Quadpole or dipole speakers, but others are very high on 6.1. If upgrading to a 6.1 receiver is going to blow your speaker budget, I wouldn't bother. On the other hand, if you're building a dedicated media room and have a big budget, might as well go all out.

Hope this helps, and happy shopping!