IRC, but with a microphone.
A more in-depth look:
Mumble was the first popular program to perform the task of allowing people using microphones to connect with each other, just like they would in IRC. Mumble, because it has been around longer, has better applications for inter-connectivity uses. Various video-games use it to move people in and out of individual chat channels depending on if they are alive or dead, and what game they are in. However, Mumble is a lot harder to use, and uses more cpu and other resources from both server hosters and the individual clients themselves. Teamspeak is the new kid on the block and uses minimal cpu and other resources to function on both servers and clients, as well as being able to transfer higher quality voice communication between the individual clients of the servers.
A lot of other companies have similar systems. The few that I am aware of are Dolby Axon, and Razer Comms. Dolby Axon is unique in that admins can move people in and out of digital rooms, where there is not only blocked communications between other 'rooms', but the physical digital location of the client in that room, and the direction they are facing, is important. Dolby Axon actually uses Dolby's digital surround-sound to manipulate the sound to make it actually sound like the other people in the room are in-front-of/behind you, and facing away/towards you.
Razer Comms is basically Teamspeak, but transfers even higher quality audio between clients within the chat rooms.
I hope that answered your question, and I think... all of... any other questions...
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positivePXL [+PXL]