RealPlayer is both a media player and a plugin.
As a plugin, it's really only used for a few major files - RealMedia files, RealVideo files and the old standard, RealAudio files. File endings include .rm, .ra, .ram.
I make sure that Windows Media Player is my default for most other audio and video files instead. As a developer, I noticed a major flaw with RealPlayer. It will try to download additional files like closed captioning, even if they don't exist.
As a Media Player, RealPlayer is simply too much. It tries to install extra software, and promote many of RealNetwork's other programs. Unless you have a large RealAudio library, you don't need RealPlayer and are free to use many alternatives such as Real Alternative.