WTF is more of a contact sport, it is this kind that is seen in the Olympics.

ITF style tkd focuses more on self defense. It incoperates kicks, punches, throws, sweeps etc... WTF focuses more on the sport and sparring consist of 98% kicks. Though wtf still teaches the above in sparring competition it is full contact with no hand contact to the face.

2 completly differents things born from the same one.

WTF is more of a full -contact and ITF is a semi contact both teach self defense but WTF is more useful in street fights but ITF for fights between friends when you don't want to hurt the other.

ITF  is "real" taekwon-do.. WTF is something else entirely..

ITF is more discipline to yourself and to others.  It teaches you to push harder for your goal.  WTF seems more relaxed in my opinion.  I've done boths styles for years, and I've always asked myself why did I do WTF because to me it's a joke.  They do this attack then you need to counter it with this movement, why do this movement when this movement is more affective?