FaceTime first allowed you to video chat between iPhone 4s. Then Apple added iPod Touches to the mix. Now? The Mac got on board, allowing you to video chat between any iPhone, iPod Touch and Mac.
The software is pretty straightforward, pulling your contacts for the buddy list and using your computer's built-in webcam. If the person you're talking to is using a mobile device, and they turn it to landscape mode, the window on your Mac automatically rotates. It seems pretty elegant, and it should make FaceTime on mobile devices a lot more useful.