Zombie Genocidest
This achievement is a clever reference to another game. Specifically, it's based on the Dead Rising achievement Zombie Genocider, which asks you to kill the same amount of zombies as the population of the town the game takes place in: 53,594. However, Valve is one-upping Capcom is saying that you have to kill 53,595 zombies, one more than the amount required by Dead Rising. As for getting the achievement, it's relatively simple: play a lot of Left 4 Dead! Both Special and Common Infected count towards the achievement.

Cr0wned
This achievement is one of the easiest to get, but only if you know about how to do it. In short, you must run up to a Witch and shoot her with a shotgun at point-blank range in the face before she can attack you. If done right, the Witch will die instantly, getting you the achievement and ridding the game world of one more Witch. As a bonus, this is a great technique that you will find yourself using later on in the game if you perfect it. There's a good video showing how to do it on YouTube, so give it a look.

Safety First
This is one of the hardest achievements to get in the game just based on how Left 4 Dead is played. In short, no player can take any damage from anything another player does. This includes walking into Molotov fires, pipe bomb damage, and the regular shot-in-the-back kind of friend fire. For kicks, try attempting this achievement on Expert, where a single shot from almost any weapon means a survivor is downed. Fun times!

Unbreakable
There's a misconception about this achievement. People think that you must not heal at all the entire campaign, and that is completely wrong. First of all, you can still use pain pills, which are an effective way to do healing short term. Second, other players can still apply healing items to you, so get your buddies to heal you up when you get torn apart. You'll get the health and this achievement!

Man Vs Tank
Another easy achievement that is only hard if you don't know how to do it, Man Vs Tank demands that you kill a Tank in one-on-one combat. While this seems like a daunting task, there are two catches to this: you can still take damage and it can be on any difficulty. Hop into a normal game with 3 buddies and play through to the Tank. Have them clear out roaming Infected that might interfere with the fight while you fight the Tank. The best way to fight a tank single-handed is to use the semi-automatic shotgun, as it deals the most damage over the shortest period of time.

Akimbo Assassin
This achievement is fairly simple as well, although can be extremely stressful. You must only use pistols, akimbo or otherwise, through the entire game. No primary weapons allowed. No grenades allowed. No mounted weapons allowed. This counts for bashing with a primary weapon as well, but not for bashing with the pistol(s). Everybody doesn't have to do it, though, so feel free to have your buddies use regular weapons while you use your pistol(s).

Outbreak
This achievement has provoked more discussion than any other achievement in Left 4 Dead due to the ambiguous wording. In short, Outbreak involves you catching a virus from Boomer vomit. This virus is transferred between accounts, so can only be gotten through another player that already has the achievement playing Boomer. When they vomit on you, you'll get the Outbreak achievement and can pass on the plague! However, according to Valve, almost half the player base is infected, so it might be an achievement to not get Outbreak at all!

Nothing Special
Quite possibly the hardest achievement in the game, Nothing Special requires no survivors to take any damage from any Special Infected during an entire campaign. This is especially hard in the face of the Tank, who usually deals tons of damage to the survivor team. There's also a special rule here: damage done to a survivor by zombies attracted by Boomer bile counts as damage from a Special Infected. However, Smokers and Hunters won't damage the player initially, which allows players to knock them off before the achievement is ruined. You should just avoid Witches when doing this achievement, as they are nothing but trouble.

Barf Bagged
This achievement is a relatively simple one, but with one catch: you have to coat all four survivors by vomiting, not with your explosion. This changes it from being a simple suicide run into something far more difficult. You must catch survivors in corners where you can vomit on all four, and manage to catch them off guard as well. It's a lot harder than it seems because of that simple catch, but with a little perseverence, is easily gotten.

Stomach Upset
Another extremely hard achievement, the entire team must avoid being covered in bile by a Boomer for an entire campaign. While the achievement says you must avoid only the vomit, it includes the bile flung by the explosion as well. Naturally, this is best tried on a Co-op game so that the Boomers don't do suicide runs at the group in an attempt to coat them. Trying it in Versus is downright impossible thanks to the much smarter Boomers.

The rest of the achievements are pretty self-explanatory and can be acquired in normal play of the game. If you are curious, you can check out how much of the player base has gotten a particular achievement through Valve's stats reporting, which gives you a rough guess on how difficult or time-consuming a particular achievement is.