The term extension means two completely different things depending on the Operating System.

For Mac users, an extension is a program that extends the system’s capabilities. Extensions include drivers and other enhancements. They reside in the Extension folder and are loaded into memory when the system starts. Mac extensions are the counterpart to the CONFIG.SYS file in the PC world.

For Windows, DOS and many operating systems, other than Apple, the extension is the part of the filename that follows a period. In DOS and before Windows 95, an extension could only have up to three digits. Although more current Windows operating systems like Windows 98/Me/XP can have longer extensions, the extension is usually kept to three characters as a matter of convention.

This extension indicates the type of information that is stored in the file. For instance, Word documents end in .doc, executable files end in .exe, and zipped files end in .zip.