For a little while now ive been wondering how big name server hacks go down, such as fbi, nasa stuff, even just corporate sites and stuff. How do they actually break in? I hear "Exploit" everywhere, but what do these exploits actually exploit, i mean, i know they exploit bugs, but what kind of bugs?

I just don't see how they're hacked other than getting bad/easy user+pass combos (admin:admin, guest:guest etc), but what else? I think you'll get my drift now.

I don't want to know this to do it myself, i just want some clarification..... thanks in advance.

(PS im not a total newbie just to this kind of hacking)