It can, however, also be used just as an ordinary Voice over IP program to talk to other people over the Internet for free. It comes in both client and server versions.
There currently exist two major versions: TeamSpeak 2 and TeamSpeak 3. TeamSpeak 2 has been around for a few years now and is regarded as very stable, whereas version 3 of TeamSpeak has only been published recently and is still very buggy (it's still in beta). That is why I will only cover TeamSpeak v2 here.
setup.shthat is inside the compressed file and a graphical installer will guide you through the installation. There is no need for aditional libraries as all necessary shared objects are shipped with the program.
Basically there are two options when using a microphone/headset. Either your friends will only hear you when you press and hold a certain key, or you set a certain input volume level which when exceeded will make your friends hear you (so that, for example, your friends won't hear you eat potatoe chips but when you say something they will hear you).
You could create your own TeamSpeak Server (you would have to download the server version) or you can connect to some of the public servers. You can find some that are free to use by clicking on the 'Web Server List' tab in the Connection window. Or you can find one by just googling.