December solstice is when the sun reaches its most southerly declination of -23.5 degrees, when the North Pole is tilted 23.5 degrees away from the sun. Depending on the Gregorian calendar, the December solstice occurs annually on a day between December 20 and December 23.
This year it was today, as all places above a latitude of 66.5 degrees north (Arctic Polar Circle) are now in darkness, while locations below a latitude of 66.5 degrees south (Antarctic Polar Circle) receive 24 hours of daylight.
For an observer in the northern hemisphere, the December solstice marks the day of the year with the least hours of daylight. Those living or traveling north of the Arctic Circle towards the North Pole will not be able to see the sun during this time of the year.
Today was the Winter Solstice, which is the day that marks the shortest day of the year. Sure this means we don't get a lot of sunshine today, but on the plus side the Solstice marks the time when the days begin to get longer. However, the actual shortest day of 2012 will be tomorrow.