Steam is designed by valve (the peps who made Half life and CSS). Steam is a program that is designed to distrubute games via the interent through one portal versus having to go through websites. But in order to play these games you have to sign into an account which has retail games registered to it. This makes bootleg copies of games very difficult to use, you NEED a valid cd key registered into a steam account before it can be played.
The wonderful thing about it though, is that you can download and install directly to ANY computer with interenet access. In speaking of which I installed my friend's copy of counter strike 1.6 (I only have source not 1.6) onto a computer last night when he let me use his account (he has two, one has half life 1 platinum pack and the other with half life 2). Didn't need to bother getting a copy of the disk or anything.
Now valve is making all their games integrated into steam, demos included. It's worth it if you ask me, as long as you don't have dial up interent it's great. The only advertising I see is for new steam games that come up in a window which has the option of being turned off I believe. If you haven't heard valve is releasing a free trial of game this weekend.
Steam at first was really crappy IMO, but they definatly tweaked it enough to be a good program. If your not a friend of background apps, your going to have to live with it, but it shouldn't do anything to a modern cpu.
However, I still have a grudge with the integrated menu system in valve games, servers on your favorites list disappear way to often. I now just use xfire or just write down the IP. Steam can be an annoying POS sometimes, but 95% of the time it's fine if you ask me.