Valve wants some of that sweet, sweet livestreaming coin, and step one of its plan is kicking off a beta for its newservice.
If you and another player both opt-in to the beta, you can start watching each other play games at the click of a button.
The first time you connect like this you'll be presented with a few options for your privacy settings, but apart from that Steam pretty much handles everything else. Select "Anyone can watch my games" and your broadcast will be visible on the game hub for whatever you're playing.
Currently the only browsers that support Broadcasting are Steam Client, Google Chrome and Apple Safari, and the service is pretty much restricted to friends. Still, it's a bold and seemingly pretty nifty move into the livestreaming market by Valve.
It will be interesting to see if the company takes this further, adding more robust support for recording and stream promotion - though there's a public streaming option, currently this is clearly designed for quick and easy streaming with friends. It'll be a lot of work on Valve's part to pin back, which currently dominates the market entirely.
You can read up on Steam Broadcasting in.