As the name suggests, it's a zombie game. You can select from a variety of modes, solo or multiplayer, for me, this game is a great time killer especially on sandbox.
Multiplayer: It's nice to have but has its downsides, most players have terrible connection so expect the ping to be high.
Sandbox: There is almost nothing to say wrong about it... except that there is a entity limit, don't expect to recreate some scenario throughout the whole map, your soldiers will only be able to cover a small portion of the map. Well you probably could put soldiers up to decent proportions but you will only be able to spawn like 4 zombies.
Overall a good game, I would recommend trying it out.