If you like the look of the person in the other box and they like the look of you you can stay and chat.
Unless you stop for introductions, you have no idea who it is that you're looking at, and neither does your partner.
The New York Times described Chatroulette as "akin to speed-dating tens of thousands of perfect strangers some clothed, some not".
Others have variously described the site as "enthralling" or "disturbing".
It started out as an experiment by a bored Russian teen, and it quickly spread among the global youth culture through dorm rooms and chat rooms.
Now, it's about to explode into the mainstream, once again redefining what's possible in web-based communication.
It's called Chatroulette, and if you have a webcam and kids in the house, you are likely to come face-to-face with it very soon.
Created just three months ago by 17-year-old Andrey Ternovskiy, Chatroulette drops you right into the living room of a succession of total strangers from around the world using nothing more than your computer's webcam and microphone.
You can see them and, yes, they can see you live, unfiltered and unannounced.
Chatroulette's home page in fact its only page is a model of simplicity, just two empty boxes one labeled "Partner", the other labeled "You".
When you press the Play button above the top screen, your webcam is activated and you see yourself in the You box.
At the same time, Chatroulette randomly connects you, Skype-like, to a complete stranger.