File extensions are used to indicate the format of files so operating systems know what to do with them.

A file extension is part of the file name and uniquely identifies the type of file, also referred to as the format.

When your instructor receives your E-mail and sees your file with a.docx extension. He knows that you are sending his a word document.

Type Type of File Extensions

§  PDF               Portable Document Format
§  PNG              Portable Network Graphic
§  EXE               System file or Application file
§  RTF               Rich text format
§  GIF                Graphic Interchange format