Teamspeak 3 is a tool that allows you to talk to the people you are gaming with across the internet.  We use it for communicating with each other while in game.  The server and client are both freely available, and all you need outside of that is a computer, speaker and a microphone.  Ideally a headset is preferred to cut down on echo and background noise.

Half the fun of gaming online is hearing that evil giggle or disgusted groan, so we highly recommend this if you decide to join in.