Left 4 Dead is a zombie game made by Valve. It's also a Steam game. In Left 4 Dead there are like 4 different levels that you can play at. Each level has around 4 or 5 sublevels or checkpoints. Left 4 Dead is about four humans that have to fight their way to spots where other humans can come and rescue them. You can weild a pistol / 2 pistols, a weapon (rifle, shotgun, etc.), health kit, pills, and some type of explosive at once. The different weapons are a pump shotgun, auto shotgun, a sniper rifle, a rifle like a m4a1, and I think another rifle that has a scope you can use. When one of your teammates gets taken down, then you can help them back up, but that teammate will have lost a good bit of health. There's different boss zombies that you may run into during your gameplay. Then of course you have your regular zombies.
What I thought was really cool about this game is that every time you play it, things are different. Like for example: There might be a lot of zombies in one place at one time, then the next time there's none. That goes the same way for weapons, explosives, and health.
When you play online, you can either play campagin mode with other people where you just simply play the levels just like you would by yourself. Then there's Versus mode where some people can be humans, while others can be zombie bosses.
Even though this game is short, it still is a really great game. Also, I forgot to mention that the quality is very high and real looking. So, you should go buy this game.