What exactly is a hacker and what exactly do hackers do on Battle.net?

Well since Battle.net uses unique codes they are probably hacking to get unique codes or something, or perhaps they are trying to change player details.

the open part of battle.net for diablo is filled with cheaters and people that kill your character, so I've heard

all of open battle net it filled with cheaters not just the diablo part, i generally try adn stay away from to much online play with big games like that, i prefer play between friends for fun (as it should be)

For Battle.net hackers are the people that run programs they downloaded off someone's website that allows them to go outside the normal rules of the game. Examples:
Diablo 1 - Kill in town hack, item duplication
Starcraft/Broodwar - Mini-map hack to see unknown areas.
Diablo 2 - Item duplication

yes, I quit Starcraft because of all the map hacks floating around, I can't understand those guys. But all games with big communities got clan-servers where cheating can be monitored better.

hmm i had a clan in open battlenet and we NEVER had problem with cheaters....we had one member that we allowed to be a cheater and we then happelly kill the cheater or so them endlessly fighting each other untill something happens....
(2 immortals fighting is BORING!)
But me as clanleader i felt proud.
I had happelly times as clan leader of SLAUGHTERclan
:) but now im stopped..its getting too boring :(