This is called Light Painting. Basically, it is the amount of time the Shutter is open during which it captures all the light activity. In order to take this picture, you need to be in a dark place, set your camera to a shutter speed between 1 - 30 seconds. Once you press the Shutter Release button, you just need to start writing in mid-air with a source of light (preferably a torch) and you will get your desired painting! :-)