Valve's In-Home Streaming idea for Steam is a sound concept in theory, somewhat along the lines of what Sony is doing with Remote Play.
For those unfamiliar, the system allows you to stream games from your "main" gaming computer to any other computer in the house, thereby essentially allowing you to access all your games from any system, regardless of its technical prowess. This will be particularly useful for those who, like me, have a gaming PC hooked up to the main television in their house -- using In-Home Streaming, you can continue playing games from your Steam library even if someone else is using the television for something else.
There's a few considerations and restrictions, however, and Valve have outlined some of this additional information in a short FAQ posted on the official Steam Community for the service -- which you'll need to join if you want to be in with a chance of joining the beta, incidentally.
Valve is keen to stress that this isn't like services such as OnLive and the like -- you're in full control of the hardware on both ends of the equation, and it's designed only for streaming between devices on the same network, not over the Internet. In theory, this should allow for a more stable, high-quality connection between the computers and indeed anecdotal reports of those using Remote Play between their Vita and PS4 suggests that this is the case -- Internet Remote Play is somewhat laggy and provides poor quality visuals, while streaming over a local network provides a much more reliable experience.
Valve does say that Internet streaming is not currently supported, however, implying that it may be an option they're considering for the future. In the meantime, this being PC gaming, you can more than likely count on some clever modder coming up with a way to use the existing framework to stream over the Internet.
One important consideration when using the In-Home Streaming system is that no-one else will be able to use the "host" computer while you're streaming it. Valve's explanation for this is so that the host can dedicate all its resources to running the game as efficiently as possible, and that there will be no conflicts between inputs on the local machine and the remote client.
There are still a few unanswered questions at present, most notably what the specifications on the client machine will need to be in order to get good performance. Since it's only streaming video rather than rendering the game in real-time, its specifications can theoretically be considerably lower than your "main" gaming machine, but what will be needed to minimize display and input lag? And will you be able to stream non-Steam games as well as those in your Steam library?
Streaming and other disc-free ways of playing your games when away from home are still relatively new technologies that all the major platform holders are experimenting with in various ways. Sony appears to be furthest ahead in this regard with Remote Play between Vita and PS4 -- and, should it ever see a release over here, Vita TV and PS4. Microsoft, meanwhile, takes a different approach -- rather than streaming, you can access your digital library of downloaded games from any console you sign into. If the console you sign into doesn't already have the game installed, you'll have to download it first -- and it will remain on the console in an unplayable form after you leave if your friend doesn't also own the game. Microsoft's approach has the benefit of actually running the game natively on the console rather than relying on potentially unreliable streaming video, but given the size of many Xbox One downloadable games, this could play havoc with those who have bandwidth caps from their Internet service providers.
It remains to be seen how well Steam's In-Home Streaming system will work, but at least you'll be able to try it out and use it for free -- Valve has no intention of attaching a fee or subscription to the service, since it's all being handled by your own network rather than going via their servers.