Your computer has many different types of files on it, and each one has its own file extension. A file extension is a three- or four-letter identifier found at the end of a file name and following a period. These extensions tell you about the characteristics of a file and its use. In this lesson, we'll go over some examples of these extensions, as well as how to determine a particular file's extension.
Some operating systems hide file extensions by default to reduce clutter. It is possible to show the file extensions if they're hidden. Click the links below to see how to show file extensions in Windows and OS X:
You can also usually tell what the file type is by looking at the file's icon. For example, the Word document looks like a file with a W in the corner, while an Excel spreadsheet looks like a file with an X in the corner.
File extensions also tell your computer which applications to use when opening that file. Sometime you may want to use a different application to open that file.