To install it simply download the deb package from the above page and install it using dpkg.
sudo dpkg -l steam_latest.deb
You may need to fix up some dependancies
sudo apt-get install -f
If you use a 64 bit distro you may also need to install the 32 bit libraries;
sudo apt-get install ia32-libs
Once you are done, simply double click the Steam icon on your desktop or find it in the Games section of your Applications menu.
Be aware that the first time you run Steam it will need to download the bulk of the application from the Internet (steam_latest.deb simply sets up the repository and acts an installer)
I only have 3 games that work so far but that's a start.