TeamSpeak is an application which allows its users to talk to each other over the internet and basically was designed to run in the background of online games. The voice quality and hereby the bandwidth usage is configurable and can be low for modem users or as good as normal phone calls for better connections. Still the client's maximum bandwidth won't exceed 25.9 KBit/s (=3.3 KB/s) upstream.
Also TeamSpeak uses as little latency as possible, which makes TeamSpeak one of the best online gaming communication programs.
TeamSpeak has a lot more features than these basics, but even unexperienced users don't have to be scared. As TeamSpeak is divided into a server and a client application, advanced users can set up a TeamSpeak server, which is so to say the meeting point for the clients. This means that beginners can just fire up the client, connect to a server and talk to their friends. After the first small steps they will discover a whole variety of features that give TeamSpeak a high rating when it comes to voice chat tools as well.