To dork out a bit:
PCM Stereo = (pulse code modulation) uncompressed digital audio (with two channels of sound interleaved together, to get two channels of sound at once, as opposed to two separate mono channels). This is what a CD is, this is what .wav files and .aiff files are. It is meant to accurately recreate an analog waveform, whereas Dolby Digital does not because it takes out stuff most of us won't notice. Variations of quality exist; a CD is 44.1khz 16bit, the high resolution DVD-Audio format is 192khz 24bit. Other variations of PCM are PWM and even DSD (only used on Super Audio CDs).
Dolby Digital Stereo/5.1 = compressed digital audio, stuff is taken out to reduce the size required for storing the sound. It was created for playing back film soundtracks. They fit the Dolby Digital information between the sprocket holes on the film. If you were to play Dolby Digital without a Dolby Digital decoder it would sound like bursts of white noise, sort of like a loud water sprinkler.
AAC would be like Dolby Digital, it's compressed; they take out stuff most of us won't notice to reduce the size (for downloading).
hope that helps
