Blizzard is a large, widely popular
game developer. Originally, they developed primarily for the
Mac, but like all
Mac-centric developers, they eventually realized they must either become a
Windows-centric developer, or accept a permenant role as a
small time developer. Blizzard did the former.
Blizzard is reponsible for, among other things, Warcraft, Warcraft II, Diablo, Starcraft, Diablo II, and various expansion packs for those games. Their next big thing is going to be Warcraft III. All Blizzard games have been multiplayer in some facility. They use Battle.net, a proprietary chat-and-gaming service, as the catalyst for the multiplayer modes in all their games.
However, Blizzard also has a reputation for making some super-ultra-horrible mistakes. When Diablo II was released, the incredible load forced battle.net to its knees, giving many people a bad first impression of the game. Diablo II also has a history of crashing, like a lot, and stuff. Blizzard also tends to slip on release dates, revised release dates, revised revised release dates, and revised revised revised release dates.
Their Mac department is especially laughable, these days. A friend of mine, analyzing Diablo II with MacsBug, found that several of the functions were directly named for Win32 API calls. That's right - they rewrote the Windows API to port Diablo II to the Mac. Ugh.
So, most people have a sort of love/hate relationship with Blizzard. They have good ideas and, sometimes, good products. But in other respects, they really suck at fixing bugs, keeping their servers up, and so on. But none the less, everyone knows them, and everyone has some degree of respect for them.