Bethesda have decided that the DLC for Skyrim is going to be come in a small number of hefty chunks, as opposed to lots of little ones.
Personally, I prefer this approach (as much as I like anything about DLC). The first DLC/expansion is entitled Dawnguard. Spoilers abound below.
Dawnguard will initially be released for the Xbox360, with the PS3/PC versions coming a bit later. It introduces the Volkihar vampires and the Dawnguard, a group of vampire hunters who appear to have nothing to do with the Vigil or Meridia, despite earlier speculation about the name.
These two factions will offer the player a choice. Join the Dawnguard and you get a new fort to live in and crossbows, join the Volkihar and you get a castle to live in and the ability to transform into a vampire lord. The two factions will battle to defeat one another, though how involved this will be is uncertain. The Civil War in Skyrim took quite a while (unlike some other questlines, such as the Companions, which were pretty brief).
There will also be access to a new plane of Oblivion: the Soul Cairn, which is filled with the undead.
Players will also have access to new dragon shouts as well as new skill trees (for vampires and werewolves respectively). There will be about a dozen new enemy types, although it sounds like they'll be mostly modified versions of existing foes.
I really like the sound of the Dawnguard and Volkihar, but I'm also pretty annoyed with the introduction of two new skill trees. That's core gameplay, and should've been included with the game proper. Extra spells, shouts, armour, weapons and so on are welcome additions but the perk trees are not something that should be used as add-ons, I think.
The DLC itself looks like it'll be pretty cool, but I won't be buying it myself. I'm going to wait and see how much the Game of the Year (GOTY) edition is, and how good the extra DLC looks and consider buying the GOTY edition if it looks a reasonable deal.
For more information on Dawnguard specifically and the Elder Scrolls/Skyrim more generally, it's well worth visiting the excellent Unofficial Elder Scrolls Pages here: http://www.uesp.net/wiki/Main_Page