File extensions are the second part of file names that come after the period. Windows by default only has file extensions appear on file types that it does not know. Known file extensions include .doc or .docx for Word documents, .txt for text files (ones that contain only texted that does not have markup codes like bold and underline), .exe for executable file.
The Windows default on hiding file extensions results in people appearing to have violations of the rule that two files in the same folder cannot have the same file name. For instance Dwight-file.exe and Dwight-file.exe would both appear as Dwight-file but with different icons beside them. If we show file extensions then both the files would appear as full names.
To change how they appear on the whole computer click Windows Explorer on the Start screen in Windows 8 or 8.1. Then click The View tab on the ribbon at top. Put a check mark (by clicking box) in the File Name Extensions option on right on the ribbon.