It is officially here. December 21 was the start winter for all those in the Northern Hemisphere and the summer for everyone down under. The Earth titled on its axis so that the sun was over the Tropic of Capricorn shining most of its light and heat on the Southern Hemisphere. A bit of trivia for any interested, the Earth is much closer to the sun at this time of year than it is at in June. The axis of the Earth is that powerful in the say of the seasons.
Geography@About.com has good information on the Solstice and the Four Seasons in general.