Other sites are Facebook, Myspace and Bebo.
It basically means 'What the hell' and is mainly used by teenagers who do not wish to curse outright or think it is cooler to use an acronym.
It shows many different emotions, depending on what context it is placed in. It can show surprise, happiness, anger and confusion.
Girl: "I bought you a handbag, it's Gucci."
Mum: "I'm breaking up with your father."
Boyfriend: "I don't love you anymore."
Boy: "I love you."