In Diablo terms, hackers means cheaters.
They are not real hackers in any sense of the word. They are just people on Battle.net who use cheat programs for equipment, super high stats, god mode (invincibility), auto-kill, etc. However as bad as that may sound, even if you encounter one of these "hackers", there is no threat to your computer or your Diablo character. They cannot alter anything on your side, although certain cheat programs will be able to scan your character and report back everything about your character, from stats to gear.
There are also the PK'ers (Player Killers) to worry about. They are usually also cheaters, but not always. In Diablo there is really no point in killing another player unless you enjoy ruining someone else' experience, which too many people obviously enjoy doing

But fear not, if you are killed by another player all you drop is your ear (acts as a trophy) and some gold. However, all your equiped gear is dropped when you're killed by monsters, so be careful who you party with because a greedy companion could help himself to your stuff if you're not careful. But these people should really be no concern to you. Just remember one basic rule: only play with people who you respect and trust -- and that pretty much eliminates everyone who you'll meet on Bnet
Here's a link that might help you learn the ropes a little better:
http://www.arewehavingfunyet.com/diablo/newbie2.html