Once upon a time there was a game and people made a mod for that game and then some people who might have been the same people made that mod into a game of its own and it was called Killing Floor. Then Syl's boyfriend (I know, who wasn't disappointed to learn that?) bought me this game and together we shot zombies and it was good.
The basic gameplay is this: mutant zombie people come after you and your team and you kill them.
Starting out, you'll have a pistol and a few grenades. Killing zombies gives a bounty for a few pounds (it's in Britain, so they use weight as a measure of wealth, which is why the US is so rich), with much bigger bounties for more powerful enemies. After a wave has been eliminated, which can be anything between a couple dozen to a couple hundred, depending on the wave and number of team members, there is a minute of peace. During that time you can buy guns, ammo, grenades, and armor from the trader, who spawns in a few possible locations on each map.
As might be expected, more expensive guns are more powerful, though it's not a matter of "buy the biggest gun." There are variable weights, so you may be unable to carry everything, or even much at all, maxing at two of the larger guns and maybe a spare pistol or melee weapon. I've had fun trying to figure out the loadout that works best for me.
On top of that there is a perk system. Players can pick from one of seven perks, which make them better with the various types of weapons and make them a bit cheaper. These are leveled up by some aspect associated with the perk, such as dealing lots of explosive damage as a demolitionist or healing as a medic. The points can be gained while using any perk, so in my case, since I often use the inexpensive pistol to get headshots on weak enemies, I leveled up the sharpshooter perk at least once while using a different perk.
There are a dozen and a half or so standard maps to choose from, with a half-dozen which are particularly popular. One of my favorites is called Mountain Pass, which takes place on a mountain with a road going around it, which gives a nice mix of a fairly-safe, high-visibility road area with a more close-in, riskier forest area higher up, which is where much of the spare ammo can be found, sometimes making it a necessary risk.
Oh right, you can run out of ammo. It's not a constant problem, but if you're shooting aimlessly and without aiming, you're going to run into problems. This is why I've gotten into the habit of using the pistol, because it's inexpensive to restock and it saves my expensive ammo for dangerous targets. I like that aspect, that bigger, more expensive guns are not always better or always the best choice.
West London is probably my second favorite, which I suspect is due to it subconsciously reminding me of dead-side Stratholme (though now the sub part is ruined). Of note is a tunnel where players often take cover, with a team watching each end of it. It sounds very secure, with certain choke points and responsibilities... until one side gets hit hard, the other side needs to help, and then suddenly the choke points aren't for zombies getting in, but players getting out.
Another fun map is Biotics Lab, which is where the mutants were created (they all have some backstory, but it's not important for the gameplay). It's a closed-in area where it becomes critical to watch everyone's back and keep an eye on door welds and side passages. I've seen some games that go perfectly smoothly, with guys on both ends keeping things safe and secure, and games where a side door bursts open and suddenly we have three dozen very angry zombies right on top of our heads. Each is a different sort of fun.
On the subject of fun, it's a fun game. You can pick the map to play and either with friends or just join a server and hope you get good people. Sometimes you do, sometimes you don't. Thankfully, there isn't really a penalty, or reward, for winning, beside satisfaction and of course achievements. While this makes victory seem a little hollow, given the frequency of defeat, it's probably for the best. It can be pretty quick for things to go from good to disastrous.
The nice thing is that either a smooth or a barely-surviving game can be fun. A smooth game can give the satisfaction of playing well and working as a team. A game where you barely survive, well that's exhilarating!
The graphics aren't anything spectacular, but they get the job done. If you do much long-range shooting, particularly as a sharpshooter, turning up the graphics is a big help.
The sounds work quite well, alerting you to nearby zombies, or just making you nervous and paranoid. I'm not sure if it's designed this way, but it often seems as if certain enemies have no directional sound when far away, but then they do when closer. So you might hear the rattling or chittering of the Crawlers (arachnid-human hybrids), somewhere, but it's not quite apparent where. Until you're certain that they are... yep, right behind you. This sound is essential for survival, since the first-person view doesn't say what is behind you: so if you're backing up, listen for the moaning or deranged whispering (the latter is a cloaked Stalker).
It makes a sort of conditioning. If you've played WoW, particularly on a PvP server, you know the reaction to the "woosh" of a rogue: WHERE IS IT WHERE IS IT AOE AOE I KNOW YOU'RE THERE COME OUT NOW NOWNOW OH GOD I'M DEAD ALREADY. Similarly, hearing the sound of a chainsaw or the snuffling of a Fleshpound (it is named for the fact that it pounds you into mere flesh) causes you to back up, while the angry ROAAAAAAAR makes you either A) Run away while peeing or B) Turn and unload a full clip while peeing. Note that the "only have to be faster than the other guy" rule is not always consistently applied.
In terms of the in-game community, I think it's generally okay. People help each other, sometimes effectively, sometimes not. They share [in-game] money when they can, since it's not any help to have a poorly-armed teammate. Of course there are the jerks, but I've not run into many people who I really wished I'd never met. For the most part I didn't care and in a few cases, I wanted to shake the hands of the people I met, because some were truly amazing players. If you don't want any of that, just play with a few Steam friends on your own server. Or even solo, but soloing isn't as much fun, and can be quite a bit harder since there is no one to watch your back.
I highly recommend this game.
P.S. Buy a level-action rifle; it's the absolute best gun for the price.