PCM (Pulse Code Modulation) is the basic digital format used for most home entertainment audio, including movies and CDs. Audio systems are engineered to process PCM to produce sound.
DSD (Direct Stream Digital) is an alternate digital format developed by Sony and used on SACDs. DSD cannot be processed by most players and receivers.
DD 5.1, DTS, TrueHD, and dts-MA are data compression codecs whose sole purpose is saving space on discs. A PCM track is fed into the encoder, which compresses it using one if those codecs. The decoder decompresses the file, turning it back into PCM. DD 5.1 and DTS are "lossy" codecs, meaning some of the data removed during encoding is not restored in the decompression phase. TrueHD and dts-MA are "lossless" codecs, meaning every bit removed by the encoder is restored by the decoder. Lossless codecs work just like zip files.
Bitstream refers to transmitting an encoded file such as DTS or DD 5.1 from a player to a receiver so that the AVR can do the decoding.