Meteor showers usually occur when the Earth moves through a cloud of particles that have been cast off by a comet. The Geminids were assumed to be from a comet, but no comet was known that could have produced them. However, In 1983 asteroid 3200 Phaethon was discovered and shortly thereafter identified as the source. It may be a chunk that was broken off of a larger asteroid, Pallas. The particles that we see as meteors may be debris left over from a collision or other breakup event.
The shower was actually quite active--despite the lackluster appearance of meteors in the image above. I stopped counting fairly early on, but saw dozens over the course of the night.
Meteors can appear anywhere in the sky, but their trails point back to a small area called the radiant. This is the point in the sky that corresponds to the direction in which the Earth is moving and plowing through the debris field.
The two bright stars near the center of the image are Castor (left) and Pollux (right). The radiant is located just above Castor. Gemini extends to the upper-right of the image. The cluster of stars to the lower-right is Messier 44, the Beehive Cluster.
