A hacker is or can be A WhiteHat or A BlackHat(cracker) or even a GreyHat
WhiteHats usually tell about problems in a system while a BlackHat does some nasty stuff.
But a Grey Hat,probally cruze's around and finds some interesting stuff.
Hackers do go onto IRC if they have time or if they are the channel moderator.
Also they have great expertise in school,and computers(and are probbally a nerd).