PCM is uncompressed (what you see is what you get)

tough question (well at least to explain)

I will brake it down to two parts: CODECs;digitalization

CODECS

PCM: PCM is the uncompressed digital sound

DD: a lossy compression, it is Dolby's old faithfull even though the CODEC was created to go up to 640kb due to DVDs limits on DVD it would max out at 480kbps (so even more lossy then it could be.

DTS: a lossy compression, it is DTSs old faithfull the CODEC has a max of 1.5 mbps, the decoder was created to be extendible (i.e. it can strip info from the newer DTS CODECs

DTS-HD HR: a newer lossy (potentialy lossless)* CODEC from DTS built on top and that uses DTS at it's core, it can have more channels, higher frequency (look at the next section) and a higher bitrate. A DTS decoder can strip out the extra info and just play the core

DD+: a newer lossy CODEC from Dolby built on top and that uses DD at it's core, it can have more channels, higher frequency (look at the next section) and a higher bitrate. Unlike DTS-HR where a DTS decoder can use the DTS core of the DTS-HD dolby is not forward compatible (so if all you have is DD+ on the disk and the player only DD then it won't work), so Dolby decided to modify DD+ for BD where the DD core is seperate from DD+ and DD+ becomes only the extrensions (so every DD+ has a DD companion to go with it) **

DTS-HD MA: it is DTSs compressed lossless CODEC, in theory it should decode to match the original PCM. Note: DTS created the CODEC with an embeded DTS track, if you don't have a DTS-HD MA decoding it will be decoded on a DTS decoder as DTS

DTHD: it is Dolby's compressed lossless CODEC, in theory it should decode to match the original PCM. Note: the same applies here as with DD+, for BD there is a DD companion and the DTHD "file" will just have the extras

* unlike DD+ where the CODEC is inherently lossy (which means loss in the richeness of sound because there is loss in data) because of prefiltering if the BW is high enough then you can in theory get a lossless track from DTS-HD HR something that you can't with the other lossy CODECs (well unless the sound is very clean to begin with)

** this is important, it is the main reason why DD+ was used a lot on HD DVD but has no value on BD, On HD DVD DD was limited like on DVD, so if you want the full richness of DD the studio needed to go with DD+ (and the extra overhead needed) but on BD since the two are seperate there cannot be an empty DD+ file, so the only use is to get more channels (7.1) or higher frequency, two things that don't happen often. Also

digitalization

maybe this should have been first, but your question was about CODECs so I thought it better to start with that. I also thing it important in the discussion of sound quality, so I added it as a second part.

sound is all around us and it is analog. In order to get it digital it must be quantifiable (given discrete values). In order to do this it has two variables

Sampling frequency: the easier explanation is to think of film, how does film work, it is consecutive still images, how often an image is taken (or shown) will affect the final outcome, the same is done with audio most movies will be 48 kHz, but they could sample all the way to 192 kHz (the max for BD). The more samples you have the more true to life the sound will be and the more exact the sound function created by the DAC in your receiver or player.

Bit depth: This is the number of bits associated with each audio sample. If we stick with a video analogy it would be the same as how many colours you can record/show. The usual values are 16 or 24 (with 20 being used a bit, but less often) the more bits you have the smaller the differences you can record (think of slicing a pie, if you slice it into 4 then the least anyone can have is 1/4 and the next size piece is 1/2 but if you slice it in 8 then one person, who wants a sliver, can have 1/8 of a piece and the other person that wants more can have 3/8)

So not all audio is the same even if it is the same CODEC, if one comes from a 16/48 master and an other from a 24/192 the quality wont be the same, and then the CODEC comes into play, so you can get into interesting questions like is 16/48 lossless better then high bitrate 24/96 lossy.