Mumble and Murmur

Mumble is a voice chat application for groups of people. While it can be used for any kind of activity, it is primarily intended for gaming.

It can be compared to programs like Ventrilo or TeamSpeak. People tend to simplify things, so when they talk about Mumble they either talk about "Mumble" the client application or about "Mumble & Murmur" the whole voice chat application suite.

Murmur is the name of the server application. In any case, if anyone talks about Murmur it is for sure the server part.
The client, Mumble, runs on Windows, Mac OS X and Linux. The server component, Murmur, should run on anything you can compile Qt 4 on.