1) A file is used to remove material from what you're working on with cuts a scratches. A file has small "teeth" and a handle that is usually made from either wood or plastic. The teeth are used to glide across the surface of what you are working on in order to take material away. Stones are similar to files in the fact that they are used to shape and smooth out a workpiece's surface, but stones are more made to polish or remove small amounts of materials. They are also shaped like files, but don't have any teeth like a file does.

2) The most common file types for metal use are flat, round, half-round, triangle/ knife, and needle files. These are used for different surfaces or for different types or jobs you need done with a file. One example is the diamond file. These are used on hard surfaces like hardened steel because regular files aren't hard enough to file down hardened steel.

3) Yes, you can use a bastard file on a metal part. A bastard file is commonly used to quickly remove material or for rough shaping. A bastard file wouldn't be ideal for a job where a smooth finish is wanted as it will come out rough. You can also use a smooth cut mill file on wood, but isn't ideal for either shaping or removing large amounts of material so would more than likely not be the best choice even though you could technically use it.