Eris is the goddess of discord and strife, sister to Ares, god of war. She spread rumors among men, just to watch the bloody wars that ensued.
Zeus was preparing a wedding for Peleus and Thetis, and because of Eris' troublemaker reputation, he purposefully did not invite her. Well, this really pissed her off, so she decided to create a little chaos. Eris thus created a golden apple and inscribed the word »Kallisti« on it, which means »To the prettiest one.« She rolled it into the wedding crowd, and three goddesses began to fight over the apple: Athena, Hera and Aphrodite became consumed in the fight, throwing food and bitchslapping each other.
Finally Zeus came in and stopped the catfight, sending the apple to the city of Troy, and a man named Paris. It was Paris' duty to choose which of these three godesses was the most beautiful, and ended up choosing Aphrodite, goddess of love.
Well, she screwed him over by grabbing the golden apple and leaving, but then fixed him up with this chick named Helen, but Helen was already married. And thus began the Trojan War, but that story's for another day.