Today is the winter solstice, the shortest day of the year in the northern hemisphere (and the longest day of the year in the southern hemisphere). From here, the days will gradually get longer and the sun will gradually move higher in the sky; this process will culminate with the summer solstice on June 21, 2007.
The solstices occur because the earth's axis is not perpendicular to its orbital plane around the sun, but is rather tilted at about a 23-degree angle. This tilt causes the amount of sunlight reaching certain parts of the earth to vary as the earth rotates around the sun. This, in turn, causes the seasons; when the northern hemisphere is tilted towards the sun, it receives more sunlight; the days are longer, the weather warmer. When the northern hemisphere is tilted away from the sun, as it is right now, the days are shorter and the weather is cooler. Today the earth's orbit is passing through the extreme point of this latter condition.
It's widely held that the December solstice "officially" marks the beginning of winter, just as the June solstice marks the "official" beginning of summer. This is a common misconception, one that even I believed until recently. Since the solstices mark the extremes of the earth's path around the sun with respect to its axial tilt, it's really more accurate to say that, from an astronomical point of view, they mark the midpoint, rather than the beginning, of summer or winter. From a meteorlogical point of view, furthermore, it makes little sense to say that winter doesn't "officially" start until late December, as most colder climates have already had snow on the ground for weeks. A good discussion about this can be read here.
The winter solstice has been celebrated by humankind for millenia. The ancient Romans celebrated Saturnalia; pre-Christian northern Europeans celebrated Yule. These festivals were supplanted by Christmas as Christianity spread but certain elements of these pagan holidays (notably the Christmas Tree) are still practiced during Christmas. It's probably not a coincidence, furthermore, that the western world marks and celebrates its new year during this time period, especially since the Gregorian calendar is based on the solar cycle. Whether we realize it or not, the celebrations marking the astronomical event of the winter solstice have historically had a great effect on civilization.
So, like the folks over at Houstonist, I wish everyone a happy solstice. It doesn't get a lot of recognition, but without it, we wouldn't have all the other winter holidays we celebrate this time of year. In fact, we wouldn't even have winter!