technically speaking, if you say "hacker" you really mean someone that is really into hardcore programming and always wants to figure shit out.
but what you mean is "cracker" and you insult the hacker by calling them that.
a cracker is someone that wants to get into a system and do devious things. usually along the lines of the script kiddies that supersodomizeme is getting at.
there are white hat and black hat crackers/hackers - the white hats will dick with a site to see what is broken, and then either report the problem and how to fix it, or will just fix it and go away.
the black hat will use the hole to their advantage and do negative things. lately it usually involves "exploiting" wide open MS boxes and changing the main index page to a website and saying that you indeed have mad skillz and are not to be fucked with - usually done so in broken english because the bulk of these dumbfucks seem to be from the netherlands or korea where apparently there isn't much to do.
phreakers got started by Captain Crunch who is an old guy that really knew the system.
but with the advent of the digital switching systems, all of the phreaking techniques no longer work. war dialing, the boxes of color, tone generators, etc.
do a search and read old issues of 2600 (that was the frequency that when played into a phone, you could make free long distance calls).
also trust that I'm more L33t than you ever will be. I can't back that up, but I don't have to.
also, trust that you likely have had sex with more women than me.