Someone who flamboyantly and dramatically exaggerates their actions. This term works especially well for individuals who are exuberantly worked up over a matter that is of insignificant proportions.
Jo Saget: OMMGGZZZZZ, I can't believe it, Bob never called me back!!!!1111 My emo tears are streaming down my face and the world is out to get me. Everyone hates me and no one loves me and I'm so emo...I'm going to go listen to some Dashboard Confessional and then go cry myself to sleep.
Some other person: Chillax, you damn homoerotic cow!