While the previous writeups correctly mention that Ravenloft is a campaign setting for AD&D, what it fails to mention is that it originated as a humble module for 1st Edition AD&D.
Ravenloft was in the "I" series of module, as I6. It was 32 pages, not including the map, and written by Tracy Hickman and Laura Hickman, the interior art was done by Clyde Caldwell. It was geared towards levels 5-7, but I think that a party of that level would have had to have been pretty large to survive. It was released in 1983 and became the best selling AD&D module.
I think most of its allure was the fact that it was one of the first modules to convince you that you were in a movie and not just in the middle of a dungeon crawl, which always felt like a board game. This was despite the fact that most of the module was a disguised dungeon crawl. It also contained a memorable bad guy, whom the players were unlikely to just roll a natural 20 and finish off: Count Strahd. (Yes, the center of the Ravenloft universe.)
I've owned this module twice, and had it "borrowed" twice, never to return. It's considered highly collectable, but if you just want to purchase the module for its contents you can pick up House of Strahd, which contains the entire module (converted to 2nd edition rules). (It is missing the cool cover art.)
There was later a sequel to the module made, Ravenloft II (House on Gryphon Hill), but that later module didn't quite seem to flow as well. (The first Ravenloft was such a hard act to follow.) Those two modules together, though, are the foundation for much of the campaign setting of Ravenloft.
And to return to the campaign setting: The limited edition of Ravenloft, the campaign setting, was just released. Reviews are good, and I plan on picking it up when the Unlimited Edition hits the shelves, so it appears that the new d20 publishers are definitely producing some quality materials.