TeamSpeak is proprietary Voice over IP software that allows users to speak on a chat channel with other users, much like a telephone conference call. A TeamSpeak user will often wear a headset with an integrated microphone. Users use the TeamSpeak client software to connect to a TeamSpeak server of their choice, from there they can join chat channels.
The target audience for TeamSpeak is gamers, who can use the software to communicate with other players on the same team of a multiplayer game. Communicating by voice gives a competitive advantage by allowing players to keep their hands on the controls.- Source http://en.wikipedia.org/wiki/Teamspeak
You can Download TeamSpeak from here: http://teamspeak.com/?page=downloads