When it comes to interesting photography techniques, I'd have to put panning somewhere in my top 3 favorite. There is so much cool stuff you can do with this technique and it applies to a lot of photography genres.
Panning is basically used to isolate a moving subject from the background, but unlike doing it traditionally, by blurring it with a long lens or using a wide open aperture, you do it by creating motion blur. You do it by following the subject's movement with your camera and shooting at a fast burst rate. It sounds a lot easier than it actually is because there are a lot of things you need to take into account. The speed at which you tilt your camera has to match that of the subject. The distance from the subject to the camera and the focal length of your lens also play an important role. But perhaps the most important setting in panning is shutter speed. Normally you would shoot a moving subject with a low shutter speed. With panning, you want the complete opposite. To get that nice, flowing look, you have to use a longer shutter speed. A good speed to start experimenting is 1/30th.
The obvious problem with shooting at low shutter speeds in daylight is overexposure. You are very likely to get blown images, even if you close down your aperture. That's where the good old ND filter comes in. A good, strong 6 or 10 stop ND filter will allow you to shoot at low shutter speeds with no risk of overexposing.
Here is a video made by Adorama TV, in which photographer Doug McKinlay takes some cool panning shots in London.