Is there a button to show Desktop on Windows 10?

Starting in Windows 7, Microsoft made "Show Desktop" less discoverable but faster to use (once you know about it). Just throw your mouse to the bottom-right of your screen and click. The tiny rectangle to the right of the Clock is the Show Desktop button now.

Why is this faster? Fitts' Law.

(You can also use Win + D on the keyboard.)

You can just hit the Windows key+D and it will do the trick :)

The QuickLaunch “Show Desktop” link is one of the features I loved about Windows 98, and after they got rid of it by default in more recent versions of Windows I always made a point of manually re-creating it. Yes, now you can click in the bottom-right corner (as stated by Alan H); but that’s not obvious, and it’s not how people of my generation learned to show the desktop!

You can create the shortcut yourself. The link is:

%windir%\explorer.exe shell:::{3080F90D-D7AD-11D9-BD98-0000947B0257}

Personally I prefer to combine this with a re-creation of QuickLaunch, so the functionality and the look is identical to the classic “Show Desktop” feature.

Other answers are still valid, but the same icon from Windows XP still exists in 10.

It is the quicklaunch bar that is hidden.

You can reinstate it: unlock taskbar, right-click on it, toolbars, new toolbar, find %APPDATA%\Microsoft\Internet explorer\Quick launch, OK then fit it wherever you want and disable its title and icon names.

If you are on a laptop, you can use 3 fingers to do a swipe down motion on the touchpad and that should work.