Generally stands for 'What the fuck'. Most people use a question mark afterwards to get the point through. Rather than using the same term for the other 'w's, who, when, where, and why, it makes more sense to actually state the word and follow it with 'tf'

Capitalization doesn't really matter.
This term can also be likened to 'What the shit?' which is more comical and has a tantamount meaning.

when you're hungover as shit after a spending a night drunk as a skunk

The World Taekwondo Federation.
WTF, holds many competitions throughout the world.

this is sometimes misread by “what the fuck” but really means “well THAT’S fantastic!” or “well that’s FANTASTIC” in two ways good or bad. plz use with caution

Of course there is only one known definition of the phrase Wtf and that, of course is Wilbur, that frigg-nugget.

Denotes: What the Fuck!

1. What the fuck.
2. World Tae Kwon Do Federation