An extension to a name of a file that helps identify the file in operating systems such as Microsoft Windows. In Microsoft Windows, the file extension is often three characters long, for example, the file myfile.txt has an extension of .txt, which is a text file. However, a file extension can also be one, two, four, or more characters long.
Tip: If you wish to list files with a specific file extension, use wildcards.
In the above example, the first file shown in Explorer is "regedit.exe", "regedit" is the name of the file, and ".exe" is the file extension that identifies this file as an executable file. The next file "RtlExUpd.dll", which is a DLL file and another example of a file.