User blog comment:ToaFairon/Top 10 Favorite game characters/@comment-957879-20100430110736

Well, you start off picking one of four characters, and you go through- with the others characters- the levels and stages, fighting off zombies as you do it.

There are "Special Infected", which are special zombies. The Tank is incredibly strong, the Witch stays in its spot, sobbing, until found. at this point she will attack you. The Hunter will "pounce" on you, and the Smoker will drag you in with its' tongue. The Boomer will vomit on you and explode, sending "Boomer bile" everywhere. This stuff attracts Hordes of zombies. Out of the three new special infected, the Spitter spits acid at you, the Jockey jumps on your back and "steers" you, and the Charger charges you down.

If you need more info, there's an entire Wiki on it. I don't think I can link it, here, though.