Windows Update is Microsoft's online update tool for Windows.  It works by sending a small application to your system that installs and runs in Internet Explorer, after your system receives the applet, the applet downloads a list of all the updates Microsoft has released for Windows.  The applet then scans your computer's files, compares the files in your computer to the downloaded list, and tells you what updates have not already been installed. You then go through the list and choose which updates you wish to install.  Layman's PC recommends installing all critical updates and service packs to both Windows and Microsoft Office.  (Windows Update and Office Update can only be used with Internet Explorer)