1. An acronym, usually meaning "What the fuck", widely used in chat programs, forums, etc.
Note: It doesn't always mean "What", it can also be Who, When, Where, and Why.
2. A phrase used when in shock or surpise.
3. A phrase used when people cannot spell out the whole three words, or just prefer choosing this WTF other than anything more appropiate.
4. Any words that fit in the acronym, WTF, such as
Where's the Food?, What's This For? , Write to File, and What The Freak.
Noob #1: WTF H3 1SH CHEETiiNG onMEHz!
Noob #2: WTF, cheetiing? o___o;;
Noob #1: YESHiES! WTF...I wonder what that means...
Noob #2: Dude...Enough. WTF do you mean WTF? -__- Way to Fly?