File extensions, for any computer files, are the three letters at the end of the file name. These three letters are preceded by a dot, to separate the file name from the extension. In the example mypresentation.ppt
the file extension is ppt
The file extension indicates to your computer, to which program this file belongs. When you double click on the file name, the computer recognizes the specific program by the file extension, opens that program and finally opens the file. In the example above, the computer would recognize that the file mypresentation.ppt was created with PowerPoint, because the file extension, .ppt is one of the extensions associated with PowerPoint.
Also Known As: extension
When Mary double clicked on the file name mypresentation.ppt
the computer recognized that this file extension is associated with the program PowerPoint, and opened the presentation in PowerPoint.