Have you just downloaded a work PDF and suddenly found the file has the extension of a text file (TXT) or even no extension ...

File extensions are invaluable information because they identify the file category (image, executable, text, etc.). See extension and hidden file. THIS DEFINITION IS FOR PERSONAL USE ONLY.

Thus, virus writers try to trick more people using double extensions, so "I LOVE YOU.TXT.vbs" is really not a .TXT file, but a .vbs file, a Visual Basic Script that is executed immediately.