One of the things that has always fascinated me about photography is how a small change in light can result in a completely different photograph. Clearly, light is a necessity for photography. Without light, there is no image, simple as that.
On the most basic level, when it's just the photographer, the camera and the world, you're using available light to record what you see. The light reflecting off of the mountain or the car or your Aunt Sally goes through the lens and onto the film or digital sensor. The patterns of light and dark are recorded and presto, you have a copy that you can print or email to your mom, or blow up and put on a billboard on Sunset Boulevard. In this stage though you are essentially just recording what you see.
If you pay attention to the results, you may start to notice differences in how the objects in your photographs appear based on how the sunlight is falling on them. If the sun is behind you, you notice that there are not many shadows visible. This makes the subject appear flat since your eye uses shadows to show depth on a flat image. If the sun is to the side of you, you will have longer shadows with part of you subject lit and part of it dark. If the sun is in front of you...holy cow! You can't see anything at all, just some lens flare and deep shadows, kind of like looking at the sun. Then you think to yourself, "huh, I wonder how I can use this to my advantage?"
It sounds obvious, but this is the basis for all lighting technique. It doesn't matter if you're talking about Richard Avedon, your 5 year old niece's cell phone pictures, the scene in Star Wars where Luke blows up the Death Star, or a TV commercial for peanut butter. Front, Back, Side, Period. Those are the choices, that's all there is to convey depth, or lack of it in a flat image.
Of course there are thousands, probably millions of variations on this concept based on things like the strength of the light, size of the light source, color of the light, how it reflects, etc. These qualities of light are responsible for the qualities of the image, and largely why the cell phone picture from your niece doesn't really look so much like the Avedon print.
The goal of all of the fancy lighting equipment and modifiers and reflectors and the rest of it, is simply to provide control over how light falls on your subject and how the resulting patterns of light and shadow can be used to show off that subject in the way that the photographer wants.