Online play with Diablo II has been afflicted by various hacks (and cheats and dupes) almost since the beta test. Such is the nature of playing an online game that people care enough about to work to cheat at. Hacks cheat the program in some way, enabling players to gain extra benefits to themselves or to their equipment, or to rip off or kill other players in illegal ways. Other hacks attack players directly with such things as trojan horses and key loggers that allow the hackers to steal your account information.
Hacking isn't always "hacking." Many players use the term "hack" to refer to any sort of cheat or rip off, even if it's not technically "hacking" the game code. Most players are victimized by others who are exploiting a little-known bug, or practicing social engineering, or using a third party program to steal their account or items. These "hacks" have nothing to do with attacking the Diablo II game code, and in many cases aren't anything Blizzard an prevent -- after all, there's no way Blizzard can stop a player from downloading a trainer that includes a trojan horse to steal their account. That sort of thing happens all the time, and quite often the victimized player blames Blizzard for their trouble, even though they brought it upon themselves by trying to use a trainer to cheat in the first place.
We've documented and posted warnings about the hacks in Diablo II since the earliest days, and you can see that information in the Warnings! section. A few of the most common hacks are listed below.