You see these things all the time at the end of a file on your computer. For example, .txt, .html, .php, .dll, .jpg, .png, .js, and many many more. These are called filename extensions. They help your computer figure out what type of information is in the file itself. It also help the operating system choose a program to run the file with. Filename extensions are important pieces of information to keep things running smoothly on your system.