If this is your first approach to shader technology, you should know a few things before we start writing GLSL code. The differences between the usual CPU architecture and a GPU are big enough to warrant mentioning them.
When you programmed applications in the past, you were aware of the underlying hardware: it has a CPU, an ALU, and memory (both volatile or for massive storage) and certain types of I/O devices (keyboard, screen, and so on). You also knew that your program would run sequentially, one instruction after another (unless you use multithreading, but that is not the point). When programming shaders, they will be running in an isolated unit called GPU, which has a very different architecture than the one you are used to.
Now, your application will run in a massive parallel environment. The I/O devices are totally different; you won't have direct access of any kind of memory, nor will it be generic for you to use at your will. Also, the system will spawn your program in tens or hundreds of instances, as if they were running using hundreds of real hardware threads.
In order to understand this fairly new architecture, this chapter will cover the following topics:
A brief history of graphics hardware
The Graphics Rendering Pipeline
Types of shaders
The shader environment
Scalar versus vectorial execution
Parallel execution