Don't forget to mention that Steam is a royal pain in the ass if your on 56k. >
Its buggy, crashes often, downloads crap you don't need/want (hundreds of megabytes worth - even when you tell it not to), makes it slightly harder to modify games (since it hides all game assets in one big file that is its own format, and you need special tools made by hackers to get to anything), it will "preload" games you don't want, etc, etc.
Some people, those on broadband mostly, love it because it simplifies certain things - takes the thinking out of playing a game for them. Others hate it, for the reasons stated above and others.
Source is just the name of the gfx engine Valve made for HL2.