In photography, panning refers to the rotation in a horizontal plane of a still camera or video camera. Panning a camera results in a motion similar to that of someone shaking their head from side to side or of an aircraft performing a yaw rotation. Or to that of an opening door if the door stays facing one way.
You can get more information at: http://en.wikipedia.org/wiki/Panning_(camera)
Panning is a great technique and once perfected, the main subject will be more or less sharp against a blurred background. The idea is to follow the subject as it passes in front of you and continue to follow it as you press the shutter and even after the shot is taken. It looks different depending on speed, background and point of view. As well the lens you use will be giving you different results.
If you pan at the same speed as the subject it will appear sharp against a streaking blurred background. Also ensure the background isn't too light and doesn't have shapes as this can create ghostlike effects or streaks in the image. A darker background is better. Make sure you press the shutter when the subject reaches a mid point along your panning track to ensure it's in the best position and try to follow without moving up or down to prevent subject blur.
It is a technique that emphasizes subject movement by moving the camera following the subject while using slow shutter speeds to reproduce the focused subject against a blurred background.
Here some images. One I shot surprising myself at day time and the others at night at a street corner in Hanoi. Hope you like them ….