file extension 
In filenames, the group of letters after the period is called the file extension, and it indicates the type of file it is or the file format in which it was created. For example, if the filename is resume.txt, the extension is .txt, and the file consists only of simple text data. If you save a new version of resume.txt in Microsoft Word's file format so that you can play with the layout and add some graphics, Word will change the .txt extension to .doc to indicate the file's new format as a Word file. If you then convert the Word file into an Adobe Acrobat file, the file extension will change to .pdf. Many applications can open only those files with the application's own file extension; files with other extensions will either be unopenable or require manual intervention to be recognized.