I need help on the concept of overcalling

[Noodles]

When you start off playing most advise is to be aggressive,so then when I saw posts where people advise to just call this down,I got confused [img]/images/graemlins/tongue.gif[/img]
Like arent we supposed to be aggressive,I think I may have a handle on this now but help me out.
I thik you overcall when there are 1 or 2 people behind you that you think you have a better hand than but if you raise they will fold and you lose 2 bets. Also you are not sure if you are better than the bettor so it is better to call as there is a chance he will reraise,then by rasing you lose the 2 after you and have to call another bet from the bettor.
anyone got some good essays or links about this concept.and how do you know when to apply it properly?