RealServer is the software that streams RealMedia (RealAudio and RealVideo) over a network. You receive the media in real-time, and don't have to wait for the clip to download.
RealMedia is any media that is specifically created to be streamed from RealServer. RealAudio is an audio only streaming format. RealVideo is a video streaming format that includes both audio and video.
The RealPlayer client for viewing RealAudio and Real Video is available as a free download at: http://www.real.com/player/.