Photography, through simple definition is capturing a subject illuminated with light. Light, natural (sun) or artificial (flash) bounces off a subject and is relayed into a recording device such as a camera and an image has been created. With natural light, you will see how the light falls on the subject and corrections or modifications are simple. Placing a subject in the shade vs in the open sun will decrease contrast and allow much more control with the finished image.
Artificial light from a flash or strobe has some challenges. From a small burst of light generated through a flash, a subject appearance can change instantly. Using simple tools like reflectors and umbrellas, this artificial light can be modified to enhance and defuse how the light falls upon the subject. We found a wonderful video that really explains how flash and reflectors work together.