HLSL is the High Level Shading Language for DirectX. Using HLSL, you can create C like programmable shaders for the Direct3D pipeline.
HLSL was created, starting with DirectX 9, to set up the programmable 3D pipeline. You can program the entire pipeline with HLSL instructions.
The programming guide contains information about writing shaders, as well as compiling, creating and binding shaders to the pipeline.
The reference section has a complete listing of the language syntax, as well as intrinsic functions that are built into the language to simplify your coding requirements.