From Valve Developer Community
A model is a collection of points, or vertices, that form a 3D shape known as a mesh. The gaps between the points are usually filled with polygons, and these polygons are usually covered with a material.
Models are created in external modelling packages that are not part of the Source SDK. Valve currently use Maya, but there are other options. The SDK tool studiomdl is used to compile files exported from a modelling package into the binary MDL format that Source understands.
Why use a model?
- A model can have many, many times more vertices than a brush. Level of detail allows the detail to be toned down when it is too far away to be appreciated, improving performance.
- Skeleton and vertex animation
- Detailed and lifelike animation can be achieved with a model. Walking, talking, running, jumping!
- Unless specifically made to behave otherwise, a model's position, lighting and animation are all completely dynamic. They can go anywhere and do anything.
- Every instance of a model is guaranteed to look the same, and you can change all of them at once by modifying one small set of files.
- Reload all models from disc.
- Print the name of the model under the crosshair to the screen.