This game is what you'd called a "Horde Mode" where you fight off increasingly difficult waves of monsters each round, however this is co-op based and there are shops mid-game (probably what TF2 stole from them for MVM) where you can buy better weapons, body armour and so on. The higher the score the more cash and most train to get the best cash so when the boss appears you can counter him with a rocket launcher, but even then he isn't that easy.

I have found myself having a a lot of fun with this game and if the mechanic I mentioned sounds good to you I'd give it a go, however the creatures are creepy so trying to single player the missions can be terrifying and difficult.