File extension is the suffix added to the end of a file name, which tells what kind of file it is. Different file types have different extensions. For example, Image files can have extensions like, JPG, JPEG, PNG and so on. Similarly other document files have different extensions that tell the file type. The extensions are preceded by a dot. .pdf, .xls, .doc are the extensions for PDF, Excel sheet and Word Document respectively. Video files have extensions like MP4, MKV, AVI, FLV etc.