VMT
A material is a .VMT text file in a mod's materials/ folder that defines the appearance and behaviour of a surface. Its chief component is usually a texture, but it also includes information about how the surface reacts when walked on or shot, what shader(s) will draw it, what type of reflections to use, and potentially many, many other properties.
A simple example
LightmappedGeneric
{
$basetexture city44/derelict_floor_1a
$surfaceprop tile
}
This is an example of a material for use on brushes.
- It is drawn by the
LightmappedGenericshader - It displays
materials/city44/derelict_floor_1a.vtf - It has the physical and reactive properties of ceramic tile.
A material for use on models would be identical except for the shader, which would be VertexLitGeneric. Valve's stock shaders generally support the same properties (commands starting with $), allowing you to easily switch between them.
See Shader Types and Parameters for a full list of Valve's shaders, or Shader Authoring if you are interested in creating your own.