At 12.03pm GMT today, the poles of the Earth were aligned with the Sun, which is what we call the solstice. While in the Northern hemisphere, the North pole was pointing away from the Sun (it was the Winter Solstice), in the Southern hemisphere, the South pole was pointing towards the Sun (the Summer Solstice), which is why it is winter in the North and summer in the South.
The image below shows a stacked series of images taken 20 minutes apart throughout the 17th December, 2005 (within just a couple of days of the winter solstice), showing how the Sun moved across the sky during that day, from the South-East to the South-West. A timelapse movie version is also available, showing that at this time of year, while the Sun is very low in the sky, the full Moon is very high in the sky.
The Winter Solstice also marks the shortest day, although the earliest Sun-set and latest Sun-set occur on different days (the 12th and 30th of December, 2008).
There are lots of special events to mark the solstice, including a gathering at Stonehenge in Wiltshire (see photographs on the BBC News). Under the early Julian Calendar, the winter solstice actually occurred on the 25th of December, but because that calendar was not accurate, the solstice slipped to the 21st (the accuracy of the calendar was corrected with the introduction of the Gregorian calendar in 1582).
However, we still traditionally keep the 25th of December as the day for celebration, and Christmas now includes a rich mix of celebrations, including Roman (Saturnalia, and Dies Natalis Solis Invicti, the birthday of the unconquered sun), Nordic Pagan festivals and the Christian celebration of the birth of Christ (see more about the origin of Christmas at this Wikipedia article).
Whichever way you choose to celebrate this time of year, we hope you will enjoy yourselves and have a very happy festive season!