Hey hows it goin. welcome to our forums.
just for starts, steam is a program sort of like xfire, except it has its own anticheat/engines involoved with their own games.
if you play games like counter-strike or team fortress2 garry's mod etc, they run on source mod. l4d2 l4d etc also needs steam to run it. right now were pretty much in that tf2 phase so if u got a spare 20$ lyin around check it out.
i'll add you on xfire in a bit but yea were gonna head towards bfbc2 and avp pretty soon as those come out in a month or so yep. welcome to our forums.