Windows key or "Win key" or "What the fuck is that doing on my keyboard key" is a keyboard key originally introduced for Windows so it could play more musical notes. On keyboards lacking a Windows key, no-one could play a π♯ (π and π♭ could be played by holding Shift+F and Shift+F♭ respectively, but without a F♯ key π♯ was deemed unplayable), and thus Ludwig van Beethoven was unhappy. Another short-lived variant of this key, known as the "Epic Win key" was introduced in 1950 but was removed when noobs started using computers.
Historically, the addition of two Windows keys and a menu key marked the beginning of the Bill Gates-era. Initially, the new keyboards were called "Windows keyboards" but this faded once people realized Bill Gates was in league with Evil Jesus and your mom. On Linux machines the logo is burned off the key with a lit cigarette and then the key is removed with a special purpose penguin bone and mailed back to Bill Gates. Also, on newer keyboards there is a circle around this key, possibly to prevent it from spreading to other keys.
Microsoft regulates the appearance of the Windows key with a specially crafted license for keyboard manufacturers made out of the Devil's horn. With the introduction of a new Microsoft Windows logo, the agreement was updated so users had to publishing new guidelines so people have to buy ANOTHER new keyboard, in order to avoid copyright breaches. A few keyboard manufacturers have already incorporated this new design into their range of keyboards because they're owned by Patrick Stewart.
Usage with Windows
Within the standard Windows shell, pressing the Win key by itself confuses the user by opening up the Start Menu. In any other situation, the Win key is used to win. Pressing the key in combination with other keys allows mass confusion as users load programs they don't want, like Help and Internet Explorer. Pressing the key at any time during a full-screen game allows the player to automatically complete the game.
Usage with minor Operating Systems
The Win key is ignored by other systems. Unix changes it's name to the Meta key, and Linux calls it a Super key. Apple's Mac OS X uses Command key instead, thus lowering themselves to the same level of Microsoft. But they do have another symbol on it that looks like some kind of weird box thing.