A model is a specific kind of visual element in the GoldSrc and Source engines. Stored in their own separate files, models are capable of greater detail and are more dynamic than brushes. Models are used widely throughout mapping, including enemies, items, viewmodels, decorations and other players.

Sometimes models are used simply to create finer details than brushwork alone is capable of. Source mapping in particular tends to make heavier use of models in this way.

GoldSrc

Models are stored as .mdl files, and can have multiple textures and skins, animations and body groups. You can view .mdl files and see their contents (animations, body groups, textures) with a model viewer such as Half-Life Asset Manager (HLAM).

By the way, one model may be split into multiple files. Assuming the model name is modelname:

• modelname.mdl is the base file.
• modelnameT.mdl is where all the textures are.
• modelname01.mdl, modelname02.mdl, ... are the files containing the animations.

When you encounter these files, it's important to copy over all of them as a set for the model to work. On the other hand, HLAM merges them all back into the base file when you save a model file, verifiable by the file size growing. Then you only need the base file.