The game has a very simple gameplay: shoot everything that moves on the screen and reload the weapon as quickly as you can to continue shooting. The most interesting part is that the movement of your character is not strictly two-dimensional. It's more like two-and-a-half-dimensional with a little depth so you can move slightly forwards and backwards.
Moving like this, you'll have to dodge zombies to buy time in order to recharge your gun. And considering how many zombies you'll have to kill in each level, you'd better be able to move quickly.
Zompocalypse is a fairly sparse zombie game that is still very entertaining. It only has a survival mode in which you kill zombies in several different scenarios until the very end, at which point you inevitably end up as a zombie yourself.