What is the button on the end of the taskbar to clear the screen called?
It is just called the "Show Desktop Button"
"Show Desktop Button" is its name. But it doesn't just hide all windows. If you click it again, it will show them again, just as they were before you pressed it the first time. Winkey+D shortcut does the same. Basically, it toggles between all your visible windows and desktop.
Also, if you want to hide all windows, press Winkey+M. Pressing this combination multiple times will not bring up the hidden windows, you'll have to do it manually. A useful "boss-key".