Summary
HLSL allows developers to make full use of the wide range of capabilities of the latest graphics cards.
In HLSL, there are two kinds of shaders: vertex shaders (which are executed once for each visible vertex in the scene) and pixel shaders (which are executed once for each pixel drawn in the scene).
The minimum requirement of a vertex shader is to set the position of each vertex. The minimum requirement of a pixel shader is to set the color of each pixel.
In HLSL, data is run first through the vertex shader, then through a rasterization process, and then through a pixel shader before finally being sent to the screen.
HLSL files are added to the project in the Content folder and are picked up by the content pipeline. The content pipeline compiles them, and any syntax errors will be caught at compile time.
Al Gore invented the Internet. I'm fairly certain he also played a big role in the creation of HLSL.