This will probably mean nothing to most of the people on my friends list. I find that thought almost as depressing as the news itself.
Eisner’s got such a body of work behind him that I’ve never got around to reading it all, but of what I have read, I’ve liked most of. He’s the author of one of only two or three decent texts on the craft and mechanics of comics, and he produced “A Contract With God”, generally reckoned to be the first original graphic novel (a bookshelf comic that had not previously published in serialised form) in 1978. He’s been a huge infulence over the medium, specifically toward a more grown-up market, and were there any justice, he’d be the household name while Stan Lee languished in obscurity.
If you have no idea who he is, you really should check out his work.