Steam is an online gaming platform on the PC/Mac/Linux where many great games of all times are in, such as CounterStrike:Global Offensive and many others. The latest Steam client (on Beta at the moment) will now give you an option to watch your friends playing a game. That means your friends can also stream and watch the game you are playing at the moment.
Whether you are playing an online game like Dota 2 or offline game like Skyrim, friends can hop in and stream your game. It’s pretty cool to watch friends playing while you eat your dinner or perhaps when they want to show you some cool moves or walkthroughs. To start watching other people’s game, simply click on the “Watch Game” under your Friends list.
If you are scared of privacy, do not worry. You can set who can watch your games.
Steam Broadcasting is only available to Steam on PC at the moment (machines running Windows 7 and Windows 8) but it will definitely come to the other platform in the future. It will be interesting to see how fast your upload connection needs to be to stream your game to a friend (or friends, even). Most Australians are still under 1Mbps upload speed on the ADSL2+.
Source: Steam Broadcasting page