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.
This section discusses the various vendor-specific shading languages.
This section discusses ARB_fragment_program and ARB_vertex_program.
This section discusses the OpenGL Shading Language, or GLSL.
This section discusses NVidia's Cg language.
This section looks at each shading language's pros and cons, to help you decide which one is right for your project.