I'm afraid I'm going to have to disagree with a couple of the comments here. Urban Fantasy, as it's being purchased presently requires world-building and magic. Merely the addition of vampires or werewolves won't do it. The world has to have its own rules, but abide by our current physical laws (physics, geology, etc.) The fantastic elements have to be woven into our existing world in such a way that a new reality is created. They're often dark and gritty, with a "noir" feel and are most often set in major cities (hence the "urban" part). Some of the current, popular urban fantasies are:
Laurell K. Hamilton - Anita Blake and Merry Gentry series
Kim Harrison - Dead Witch Walking series
Jim Butcher - Dresden Files series
Carrie Vaughn - Kitty and the Midnight Hour
The books are either shelved in fantasy or horror, depending on the store. The publishers with urban fantasy series are wide and varied: Roc, DAW, Tor, Harper/Torch, Warner, Leisure, etc.
JMHO, of course -- based on editors I've talked to.