When used with a question mark, someone is asking What The Fuck because they can't believe that how utterly insane
When used with an exclamation point someone is says Wow That's Fantastic! This case is usually sarcastic, though because Fantastic
really means Insane
"WTF? I showed you how to type in your username and password 3 times and you still can't figure out how to log into your computer?"
"WTF! You got a promotion for sitting on your ass while I did all of the work!"