from what i know, vtf stands for "valve texture file" used for, well, textures. the packs on this site not only give you props but also textures to put on brushes ect. mdl is the model itself and (idk if this is true, pls correct me) a vmt file could be the texture for a model. I have never installed any pack from this site, i use only in game content. i can not tell you what you have to do with them sorry.
edit: i accidentally wrote vmf instead of vmt