RealPlayer can play most of the formats that WMP can, plus some of it's own (ie formats created by the RealPlayer developer) which WMP can't play.
However, although RealPlayer plays more formats and you need it for some websites which stream audi/video content only in RealPlayer's native formats, It can be somewhat of annoyance when it's installed -- always popping up nag-screens to buy the 'pro' version etc. Something which WMP never does.
In essence, if you want all video and audio formats covered, you need them both. My policy is to manage with Windows Media Player. I just avoid using websites with video/audio content that need RealPlayer.
Edited by pip22, 10 February 2007 - 10:31 AM.