Linear PCM is an uncompressed audio format. It can have up to 8 channels of audio at 48khz or 96khz sampling frequency and 16, 20 or 24 bits per sample. It has a maximum bitrate of a huge 6.144 MB/s. It's also known as LPCM.
On DVDs there are several audio formats that can be used. You may see AC3 (aka Dolby Digital), LPCM (uncompressed), MPA (MPEG-Audio), or DTS (Digital Theater Surround).
Mandatory formats for DVD differ for PAL and NTSC discs. For PAL the mandatory formats are MPA, AC3, and LPCM. For NTSC they're only AC3 and NTSC. Every disc must include at least one mandatory audio format.
HD DVD and Blu-ray also have mandatory audio formats. For HD DVD, mandatory formats are AC-3, LPCM, Dolby Digital EX, DTS, Dolby Digital Plus, and Dolby TrueHD. For Blu-ray they're the same, except that Dolby TrueHD is optional instead of mandatory.
Ogg Media also supports the standard for its codec, as does VC-1 and MPEG-4 AVC.