The idea of photography being the practice of capturing light interacting with a subject encourages us to consider several characteristics of light and how it affects the subject. Three that we must consider are direction, source and color.
First off we should consider the direction of light relative to our subject. The light might be in-line with the lens of the camera (axial light) and either illuminating the subject from the front or from the back.
Front light creates a (relatively) even distribution of light and allows full illumination of a large part of your subject.
(The Bedhouin, Canon XSi, EF-S15-85mm @ 22mm, 1/1000s, f5.6, ISO 800)
On the other hand backlight can create strong shadows and highlights. The setting sun backlit these light-rail tracks on 4th South in Salt Lake City.
(4th South Trax, Canon XSi EF-S55-250 @ 163mm, 1/125s, f13, ISO 100)
The reflected highlights off the rails and the building shadows created a range of light (the difference between dark and light) that exceeded the capabilities of my camera and I had to choose settings that captured my mood and impression of the scene.
We can also use side light to illuminate our subject. Side light gives shape and volume to a subject accentuating shapes and textures. These domes on the Sheikh Zayed Grand Mosque in Abu Dhabi gain volume and shape due to the side lighting.
(Canon XSi, EF70-300mm @ 300mm, 1/125s, f8, ISO 100)
Textures come to life under side lighting which accentuates the minor surface variations with light and shadows. While walking around the Mosque in Abu Dhabi I found this amazing relief and wanted to photograph it. Unfortunately, at the time I first saw the wall the light was axial and the texture all but disappeared. I came back later in the day when the sun had moved around to the side and captured this image where the texture really pops out.
(Canon XSi, TS-E45mm, 1/250s, f6.7, ISO 100)
Of course the light source might be at any angle relative to our subject and as it moves relative to the subject, or the subject moves relative to it, the light accentuates or masks various properties of our subject. Understanding how the direction of light affects the mood or interpretation of a subject is fundamental to expressing your vision.