WINDOWS = is the software that allows you to open and close programs like America Online and Microsoft Word with the click of a mouse button. In the old days of computing, you had to type all of your commands (like OPEN and CLOSE PROGRAM). Windows allows you to click on buttons and icons with the mouse instead. The whole look of the screen on your computer's monitor is due to Windows. Your computer would be a very expensive paperweight without Windows, because you couldn't do anything with it. For this reason, Windows is called an "operating system" or OS.
One of the competing operating systems to Windows, of course, is the Macintosh OS, which is found on about 9% of the computers out there. Microsoft's Windows is based on the Macintosh OS, but is not compatible with it. Programs that work on one will not work on the other, but there are versions of Microsoft Word and so on for each OS, and you can usually share documents between the two operating systems.