Light Painting

Photography is all about light. Light reflected from (or directly from) the subject you're shooting goes through the lens (or other focusing or non-focusing element) and onto your recording medium. There are several different ways to alter how exactly light hits your medium and what gets recorded onto it.

Light painting is one of those techniques.

Light painting can be used to illuminate a subject, add artistic meaning to a scene, or even be the subject in itself. It requires a long shutter speed and a source of light. Normally, you'll want to put the camera on a tripod to reduce any amount of movement that could attribute blur to the resulting image. For very long exposures, it often helps to put the camera on timer and mirror lockup (if your camera supports such features) to further reduce movement.

A good technique to add the light trails (as seen in the image above) to a long exposure image is to use a flashlight (with the lens portion off, exposing the bare bulb) or an LED light. Trace your light source through the air, in the desired fashion you would like your trail to show up in the image. Make sure to point your light source towards the camera, otherwise you may be just illuminating the area around you.

Light painting most often refers to painting the highlights and exposure onto the subject itself.
Whatever the light hits will be illuminated based on the intensity/length of time that area has light on it.

Both techniques can be combined! Producing spectral results as shown: