Steam is a free application that you install and allows you to buy games and play them online like Halflife Deathmatch... so you can download straight to your puter instead of waiting til EB opens on a Sat morning!.. and you can play against other people etc.. or you can play single player games and you don't need to online for that. You can buy packs, and if you have a game already thats included in the pack, then you can give it as a gift to someone else.
One annoying thing is that if someone else in the house wants to play at the same time, then they need to buy it themselves, or otherwise use your login when you're not using it.
But then things like HL Deathmatch are pretty cheap caus they're getting a bit old now. So just buy anything made by Valve software
So you can take your login anywhere, but you need to download & install the game again if you use someone else's puter.
Also you can get games quicker through Steam than waiting for them to be released in stores afaik.
And you can add friends and chat to them through Steam and have games against them like Deathmatch or Counterstrike etc.
And then if you're anything like some people I know, you'll be smashing your keyboard onto the table and clicking the mouse at a rate of 20clicks/sec to respawn, and screaming 'dammit!' and 'bastard' and 'WTF no way!' in no time