File extensions are those 3 or 4 little letters after the dot which tell Windows what type of file it is. For example:
report.xls is an Excel spreadsheet (.xls)
report.doc is a Word document (.doc)
report.exe is a program (.exe)
By default, file extensions for known file types are hidden in Windows operating systems. You can however change this setting so that file extensions are shown for files in a certain folder or for all files on your hard disk.
To show file extensions in Windows Vista or Windows 7
Open Computer from the Start menu
Click the Layout button (to the left of the Views button)
Click Folder Options
Click the View tab
Untick Hide extensions for known file types
This setting will make file extensions appear in all folders.