What is it exactly? Just Skype but better?
I hate to compare it to Skype, but pretty much. It's more like Teamspeak though.
Edit: It also has an overlay for games so you can see who's talking ingame. Also Mumble has "positional audio" where it can detect where someone is in the game and change the audio to sound like it's coming from the player.
For example, if we were playing Blockland (this is as close to VOIP blockland will ever get) and I was to your left and I said something, you would hear something from the left.