When you click on a file in Windows Explorer or in the My Computer window, Windows looks at the 3 letter file extension and then checks a lookup table to determine what program is associated with that file extension. If it finds the extension in the lookup table it uses the specified program to open up the file in question. Let's take the filename "document.pdf". The 3 letter extension is pdf. The pdf stands for "portable document file" and is associated with the Adobe Acrobat Reader program. By clicking on that filename Windows looks up the extension, finds out that it is associated with Acrobat and then uses that program to open up the file and display it on your screen.
    Why is this info important? Most issues people have opening up email attachments end up being problems with registered file extensions.