A file extension is a suffix, usually consisting of three or four letters, that appears at the end of a computer file's name after a period (dot). It indicates the type or format of the file and helps the operating system determine which program should open it. For example, ".txt" is used for text files, ".jpg" for image files, and ".docx" for Microsoft Word documents

. File extensions serve as a label that tells the computer what kind of data the file contains and which software is needed to access it. Different file extensions correspond to different file formats, such as text, images, audio, video, spreadsheets, web files, and executable programs. The operating system uses this extension to associate the file with the appropriate application, enabling users to open files correctly without manually selecting a program

. While file extensions are conventionally used to identify file types, they are technically part of the filename and can be changed without altering the file's actual data. However, changing the extension incorrectly can cause the file to be unrecognized or improperly opened by the system

. In summary, a file extension is a short code at the end of a filename that identifies the file's type and guides the operating system in opening it with the correct software.