File extensions are markers on the end of computer files which indicate to the operating system what hind of file it labels. These are a vital part of all file systems used by modern computers. File extensions are designated by a period (.) after which the extension is placed. This extension is a short string of letters (usually three).
How it works
When a user tries to open a file, the operating system looks at the file extension. It then compares it to a directory of extensions which are referred to as file associations. If the extension is present in this list, it will start the program associated with it, and pass the file path of the file that should be opened to it. Assuming all goes as it should, that program will open the desired file and doe what it is expected to. If the extension is not in the file associations directory, however, the system will not know what to do wit the selected file. All it can do is prompt he user to select a program to use or open Microsoft's website to look for the needed program. In this prompt, the user is provided with an optional check-box to "always use this program." If checked, the current file extension will be added to the file associations directory with a pointer to the selected program.
Where is it?
The file extension is always placed at the end of every file's name. However, this can be confusing and inconvenient to some, so for quite some time, the Windows and Mac operating systems have been hiding these extensions when the name is displayed for quite some time. Only when an unknown extension is read will the extension usually be shown.
How Can it be Seen?
To see the file extension, a user can do a number of things. For example, the Windows Command Prompt can be used to browse through files, and will automatically show the extensions. However, a much easer way is to simply set the system's file explorer to show them.
In most Windows systems, a user can open Windows Explorer and find the "Tools" menu. If it is not visible automatically, then pressing the Alt key will usually reveal it. This window can also be accessed in Windows 7 and later by clicking the Start button, then clicking Control Panel, then Appearance and Personalization, and then Folder Options. Select "Folder Options" and go to the "View" tab within the window which will open. In the list which will be displayed should be the option "Hide extensions for known file types." Uncheck that option and click "Ok."
In some versions of the Mac OS, including Mac OS X, users can select Finder (by opening it, or clinking on the desktop), and in the Finder menu in the upper left of the screen, select "Preferences..." Select the "Advanced" tab and check the "Show all file extensions" option.
GNU/Linux was designed not to need file extensions. Rather, it used a "magic number" within each actual file to tell the system what to do with it. However, at least some file extensions are now recognized by GNU/Linux. How these extensions are displayed depends on the specific distribution of the system, but in general the extensions which exist are by default shown in the name where they actually exist.