File extensions are identification for your file content such as .xls refer to Microsoft Excel or others spreadsheet applications. Although you also can look its icon because usually the icon represents its file content. Mac OS X by default is hide the file extensions.

To show your file extensions is very easy. On Finder menu, click Finder - Preferences menu, click Advanced tab and then check Show all file extensions check box. Close the Preferences window.
And your file extensions are shown.