Persephone is the greek goddess of spring. She is the daughter of Demeter and Zeus. A long time ago, there was no winter. Persephone lived in Olympus (home of the gods) year round, and the world was in eternal spring; harvests were bountiful, and the people were well fed and happy. Persephone was beautiful, and attracted the attention of many men and gods, inculding Hades. Demeter, however, was incredibly protective of her only daughter, and refused to let anyone marry her. Hades, like every other suitor, was denied by Demeter, and so, heartbroken, he devised a plan to kidnap her, and so one day, while Persephone was out picking flowers, she beheld the most beautiful narcissus lily. As she picked it, the ground below her opened up and from the ground Hades rode out in a chariot and kidnapped her. No one knew what had happened, save Zeus and Helios who had both seen the whole thing, but neither of them wanted to pick a fight with Hades. Demeter was distraught at the dissapearance of her daughter, and wandered the earth searching for her, and slowly neglected her duties as she looked for Persephone. Hecate guided Demeter at night with her torches. Eventually, the famine caused from Demeters neglect convinced Zeus to do something, and so Zeus sent Hermes to fetch Persephone from the underworld. Hermes did exactly this, but when they returned, it was discovered Persephone had eaten fruit from Hades garden, specifically seven pomegranate seeds. This bound Persephone to the underworld, but Demeter refused to let her go back. Eventually, a compromise was agreed upon. Persephone would stay in the underworld for seven months of each year, one for each seed she had eaten. During these months Persephone is gone, Demeter is again overwhelmed with sadness, and again will neglect her duties until Persephone returns, thus causing the winter months.
See also Who were the Gods