ARS Technica has written a handy guide that explains everything you need to know to install SteamOS on your PC, you can read it over here.
A beta version of SteamOS was published on Valve's website, the download is located over here.
SteamOS is a customized Linux distro based not on Ubuntu, as was reported prior to its release, but rather on Debian Wheezy. Valve is maintaining their own SteamOS package repository (and is using these preinstalled packages). For now, the SteamOS beta is using version 3.10.11 of the Linux kernel. Other SteamOS customizations off of Debian include a heavily patched graphics compositor to link together SteamOS, Steam games, and the Steam in-game overlay. The operating system has been configured to pull its updates directly from Valve's repository. It's also got a standard Gnome desktop environment hiding out beneath the Steam-y bits, which we'll get to.