As the name suggests, Vertex and Fragment Shaders work in two steps. The model is first passed through a vertex function; the result is then inputted to a Fragment function. Both these functions are assigned using #pragma directives:
#pragma vertex vert#pragma fragment frag
In this case, they are simply called vert and frag.
Conceptually speaking, fragments are closely related to pixels; the term fragment is often used to refer to the collection of data necessary to draw a pixel. This is also why Vertex and Fragment Shaders are often called Pixel Shaders.
The vertex function takes the input data in a structure that is defined as vertInput in the shader:
struct vertInput { float4 pos : POSITION; float2 texcoord : TEXCOORD0; ...
