The world has been overrun by nu-zombies (the kind that run fast, not the classic kind), and a ragtag group of survivors (referred to not by name but by hometown) travel across the American wasteland to the West Coast, where there are rumors that one of the amusement parks is a safe haven. In a clever touch, the main character is some sort of obsessive-compulsive type with a list of rules for surviving the zombie-holocaust that he strictly adheres to. I don't think it's been addressed before in apocalypse movies, but of course it makes sense that obnoxious type-A personalities would be best equipped to survive the end of the world.
Zombieland is not as heavy on the undead as the title implies. Though much humor and action is mined from all the walking (or, as it were, running) corpses, the zombies are often more a background detail, even forgotten for long chunks of running time. Think of it more as a feature-length version of the shopping spree sequences in Dawn of the Dead; yes, there are zombies, but it's more a comedy about what it would be like if America became your own personal playground.
Though mystified at the critical love Zombieland received on its release (you ever notice that critics always hate horror movies, unless it's a horror-comedy and then they love it), I still thought it was a lot of fun. As far as zombie comedies go, it's not a puss-filled boil on Shaun of the Dead's undead taint, but it has laughs, an agreeably cartoonish visual style, a likable cast and a handful of memorable sequences. More entertaining as a comedy and an action film than it is as horror, Zombieland provides solid fun for the casual zombie fan.