Pulse Code Modulation, or PCM, is a fancy term for the most obvious way to digitally encode audio, which is sampling the waveform and and quantizing each value. Linear PCM (LPCM) does so with linear quantization, while μ-law and A-law are variants that use nonlinear steps for more dynamic range.
Differential PCM (DPCM) is a variant of PCM that encodes and quantizes the difference between two successive sample values.
For most purposes, DPCM is a historical artifact superseded by audio codecs based on frequency transforms like MP3 and AAC. My main interest in it is intentionally degrading audio signals a la Plogue's chipcrusher.