mm
Multimedia refers to the delivery of information that combines different content formats (motion video, audio, still images, graphics, animation, text, etc).
A MM pc is one that can deliver all of the above effectively. Varying levels of graphics cards, tv outputs, large monitors, wireless broadcasting, etc are needed. It should be able to play and record all types of media and have the latest ports (usb2, firewire, bluetooth).
As anything got to do with video and audio is process intensive it usually means it will be of a higher spec as regards the basic components of CPU, RAM and disk space. Try googling for "multimedia pc" and check out what the retailers are selling, as the others have posted, multimedia is a broad term these days and getting broader so it would be difficult to list here everything it should have