Below is information I sent to a friend of mine recently. I've had a few messages from people asking me "What's this Portal game you've been going on about?", and "What's this Steam thingie?"
Some background for you before diving in the links:
"Portal" and "Portal 2" are multi-platform games capable of being run on PC, PS3, Linux (I think), Xbox, and Mac. Players on any platform can play the multi-player portion of "Portal 2" with each other.
The game is non-violent, unless you count the various AI entities trying to kill you.
"Portal" and "Portal 2" are often available on Steam for crazy low prices. One can even buy multi-packs so as to gift copies to friends. As I type this, "Portal" is $9.99 and "Portal 2" is $19.99, but I bought both of them, as a package, for under $10 over the holidays. All you need to do is add them to your wishlist, on Steam, and then you'll get an email when they go on sale.
"Portal" has no multi-player, but "Portal 2" has one that is great fun. Voice works across platforms, by the way.
I play the game via the Steam gaming network (http://en.wikipedia.org/wiki/Steam_(software) ) . Steam happens to be owned by Valve, which also created "Portal", but this is a coincidence. That is, I play other games that are simply downloaded and played, via Steam, that were not created by Valve. Steam is a free online service via which you can buy and download games and connect up with your friends, within Steam, for playing the games.
A Steam account is FREE. Several games, such as "PlanetSide 2" (hilarious review at http://angryjoeshow.com/2012/12/planetside-2-angry-review/) are amazing...and again, FREE. To get the hang of Steam, I suggest creating an account, installing their client software, and downloading a free game.
Note that the client software has a low overhead. I set mine not to launch at Windows boot and only launch manually, but it can of course be there all the time if you want it to be.
Steam often has enormous specials where games are .99 or a couple of bucks.
Portal 2: http://en.wikipedia.org/wiki/Portal_2
Official Site at Valve: http://www.valvecorporation.com/
Sign up for a Steam account: http://store.steampowered.com