Shading languages: General
All shading languages share common features and pretty much do the same thing with more or less restrictions/flexibility, for example all have vertex and fragment shaders with fixed functionality in between, all support vector types as a fundamental type and all generate interpolated fragments for the fragment program input from the vertex program output. Before delving into the details of any one language one should first understand what a shading language does in general and where it fits/what it replaces in the overall graphics pipeline.
Shading languages: vendor-specific assembly-level
This section discusses the various vendor-specific shading languages.
Shading languages: ARB assembly-level
This section discusses ARB_fragment_program and ARB_vertex_program.
Shading languages: GLSL
This section discusses the OpenGL Shading Language, or GLSL.
Shading languages: Cg
This section discusses NVidia's Cg language.
Shading languages: Which shading language should I use?
This section looks at each shading language's pros and cons, to help you decide which one is right for your project.