Here's a brief response to your questions:
1. Dolby Digital and DTS (Digital Theater Sound) are two methods of digitally encoding movie sound tracks and special effects for playback in both the theater and the home. The signal for each channel is recorded discretely, unlike Pro Logic where the surround information is extracted from the left and right channel signals.
Virtually all DVD's are encoded in Dolby Digital, and sometimes include DTS encoding as well. When Dolby Digital first appeared, recordings were done in 5.1 channel surround, but in the past year or so the format has expanded to include 6.1, 7.1, and most recently 10.2 channel sound. (The first number, "5", refers to the number of speakers used to reproduce the front left, center, front right, and rear/side surround channels. The ".1" refers to the subwoofer or special sound effects channel.)
Dolby Digital (first known as AC3) was developed by Ray Dolby (the same guy that invented the Dolby B and Dolby C method to reduce tape hiss and noise on tape recordings), and was the first 5.1 channel system devised. More recently, DTS and SDDS (Sony Dynamic Digital Sound, a proprietary system developed by Sony for use on the movies they produced) formats appeared in theaters, with DTS also offered on DVD's for home use. (SDDS has largely disappeared from use in theaters, and has never been offered as a home medium.)
The main difference, from a practical point of view, is that DTS encoding contains roughly 4 times as much digital data (information) as Dolby Digital, and therefore often sounds more accurate, dynamic, and realistic.
Dolby Pro Logic, and its most recent version Pro Logic II, is a form of surround sound that uses a matrix to create the rear/side surround. A matrix simply refers to the fact that there is ambient information embedded in the main audio signal, and this ambient information is, in effect, synthetically created from the main left and right audio channels. Dolby Pro Logic was the form of surround sound that was used in the early days of home theater, before Dolby Digital and DTS were available. Dolby Pro Logic II is similar to the original Pro Logic, except it creates the surround channel information digitally, rather than by extracting an analog surround signal.
PCM means "pulse code modulated", and this is the recording method used for CD's. The PCM signal is included on DVD's so people that do not have surround decoders can still extract and process the digital sound for the left and right front channels.
Your last issue seems to address the fact that some DVD's have both Dolby Digital and DTS encoding, but only the DD information is reproduced. To be able to process the DTS signal, your receiver or HT processor must be equipped to decode the DTS signal. Even when your HT receiver has the ability to decode DTS, you must normally specify that you want DTS sound by choosing this source when the DVD menu appears at the beginning of the movie. If you do not specifically choose DTS, the default mode is Dolby Digital.
Hope this helps.