.BIN files are the 3-dimensional models used by MTM2. They are made up of three- and four-sided "faces" whose corners are attached to points in space called "vertices". These faces are visible when viewed from one side and invisible from the other. A number of faces can be put together to form a 3D (or 2D) shape, such as a truck body. When a texture file is "mapped" to these faces, you get a painted truck body.
A program such as the MDMRE Bin Texture Replacer will allow you to remap a .BIN file to a new texure (.RAW) file quickly and easily. With this program you can apply new textures to a .BIN file with the minimum of fuss, and repaint trucks without needing to remap the body - the program does that for you. To really stretch your pants in three dimensions, however, you will need a program such as BINedit that allows you to actually edit the 3D .BIN model itself. With BINedit you can manipulate faces and vertices to make almost anything you want... if you have the patience.
Note that some other Microsoft games, such as CART Precision Racing, Hellbender, and of course MTM1, also use the same or similar format for models so it is possible to take a .BIN file from one and use it in an addon for another. It is also possible to take a model from an entirely different game, such as Carmageddon, and convert it into .BIN format for MTM2.