OGM is a video and audio wrapper format, like AVI. It is indeed related to OGG, which is just the audio part (OGG files are sometimes OGG Vorbis files, because they use Vorbis compression). The advantages of OGM over AVI include support for VBR audio (AVI can take VBR MP3, but often has problems with it) and support for subtitle streams (even multiple ones). To play OGM files on a PPC you will need to use PocketMVP, and on the desktop something like
Zoom Player. To create OGM files for PPC, use
PocketDivxEncoder or
VirtualDubMod. (I recommend Pocket Divx Encoder—VirtualDubMod is
really confusing to use with OGMs at first.)