A File is a long piece of hardened steel used to scrape away very small amounts of material with each pass. There are two main types of Files. Woods files and Metal files. Wood files often have larger teeth to grip into and tear wood. These are not good for case modding because they can easily get stuck on metal and jerk your arm, injuring you and your work. A Metal file in comparison has very small teeth. The smaller teeth allow the file to not get caught on the material you are filing.