Before we begin, note that this only available for the Steam Client on Windows 7 and 8.x for now, but Valve intends to bring it to Linux and OSX (and Vista for some reason). You must also opt-in to receiving Beta releases of the Steam Client. Beyond the currently limited support in hosting a stream, watching a stream is only possible with one of three web browsers: Google Chrome, Apple Safari, and the one in Valve's Steam Client.
Obviously, the pre-launch browser support is quite disappointing. Clearly Valve limited support of their web app to their rendering engine of choice, WebKit and its fork, Blink, and that is a step up from doing it in Flash. It is probably most disappointing for Opera, who switched to WebKit (and later Blink) from their proprietary “Presto” engine. Of course, relying on a collaboration with Google for standards support can only help so much, and it apparently did not help enough.
As for the service itself, Valve is focusing on user choice with their offering. While public streams will be allowed, you are allowed other privacy options – public is not even the default. Your stream can also be set to: only allow invited friends, allow all friends, or allow friends to request viewing permissions. By default, it is set to the last (fourth) option.
Now on to the speculation…
Why would Valve being doing this? Of course, Amazon believes it is a billion dollar business, so it is not insane for Valve to throw their hat in the ring, and hats is something they have plenty of, but I believe it might be bigger than this. This announcement follows the beta release of In-Home Streaming, back in May. Especially with the privacy options, I could see this following Sony, and its PS4 Share Play feature. Share Play allows people on your friends list to override your controller, or an extra controller if you want to play local multiplayer over the internet. These are all products using the same building blocks.
Steam Broadcasting (Beta) is available now through the Beta Steam Client release channel.