Koen Samyn, lecturer interactive programming at the University College of West Flanders (curriculum Digital Arts and Entertainment) wrote an excellent KNOL (a unit of knowledge) on HLSL Shaders. You can find the article here:
What is a shader ?
A shader ( or effect) is a program that is executed on the GPU (Graphical Processing Unit). The GPU is located on the graphics card off course and the GPU contains many cores that can execute a shader program concurrently. This allows the GPU to have high throughput (number of triangles that can be processed per second).
If for example a GPU has 4 available cores (run of the mill GPU have much more then that) each code transform the vertices of 4 triangles simultaneously. There are several shader API’s and languages that are targeted to curent GPU’s : …