That's just it, I don't really know how to classify PvZ. A standard Tower Defense game seems to be based around a set path (or checkpoints) where a number of disinterested enemies travel along towards your base. You create a massive amount of defenses along the sides of this path, which try to destroy the enemies before they can reach your base. The enemies generally do not interact with your defenses in any other way than getting shot at by them.
I suppose the best description of PvZ would really be a "Base Defense" game. The zombies will interact fully with all your defenses, munching up your delightful little plants while making their way towards the house.
There are around five individual rows on the lawn (might be more later on in the game), all of which lead directly to the house. The defenses on one row (generally) cannot interact with the zombies on other rows, and the zombies on one row (generally) cannot interact with the defenses on other rows. You therefore need to build five different defensive lines in order to prevent all the zombies from reaching your house.
Furthermore, you do not gain resources from killing enemies like in a standard TD (well, you can get "bonus points" from them, with which you can buy stuff from your crazy neighbour. Like a pet snail). You plant your defenses using solar power, which drops down from the top of the screen intermittently. However, the rate at which solar power is dropped is much too slow to fuel any real defense, so you need to plant several resource-generating plants (sunflowers, solar mushrooms, Etc.) which will periodically spew out some solar power for you to pick up. Early in the game you need to balance resource use between setting up more solar generators or placing defenses.
Another twist arrives at nighttime, where solar power does not drop from the top of the screen. Also, standard plants do not grow at night, so you need to use an entirely different set of plants, including solar mushrooms which do not produce as much solar power as sunflowers (after quite some time, they do grow up to match sunflower production however). This creates an interesting economic crisis during nighttime maps, which is slightly offset by being able to spawn weak basic defense mushrooms for free.
You will eventually unlock more and more different types of plants, which you will eventually need to manage as you cannot have access to all your unlocked plant types at once. Prioritize, make favorites, and style your "deck" to suit your style (and save a place for cherry bombs. Seriously, those things rock).
Yeah, base defense would probably be the best description of PvZ. But to find out the real charm of this thing, you just need to give it a whirl and find out for yourself. It's very easy to get the gist of, while still providing a challenge, and it keeps the relatively simplistic gameplay varied enough so as not to get dull. Overall, it's quite well made.