Dude... read the Beginner's Digest sticky in this thread - all of it. You're asking some really basic questions... which is very cool, and these are all addressed through the various posts in the digest.

Simply, aggression is used for a bunch of things... protecting your hand (AA/KK/QQ to force bad drawing odds for people looking for draws), value (you got what you believe to be the best hand and you're looking to build the pot so you get paid when you win...), bluff / semi-bluff (you've got less than your representing and have good odds of taking a hand down that you'd likely lose at showdown...) and a whole lot more.

Thoughtful aggression is good, and the worst thing you can do is play scared -- at a level where your bankroll or mindset are going to keep you from being aggressive. The BR thread in the Beginner's sticky talks about this too... GL!