I have a few questions about Steam. When I buy a game from steam is it mine forever
It's not yours at all. You merely have a non-exclusive, revocable license to use the game. That's true for most commercial games though.
as in I can install in as many times on as many different pcs as I want?
Yes, so long as the following is true:
1. Steam stays open
2. You don't get banned from Steam.
If you get banned from Steam you lose all your games and if Steam closes, obviously you can't use it. Oh, and while you can install it as many times as you want on many different PCs you can only use it on one PC at any given time.
Im looking for some older games to get currently I plan on getting The Oddbox since it has 4 Oddworld games for 25$
If older games are what you are after, check out http://www.gog.com/en/frontpage/. No DRM means that you never have to worry about not being able to play them.