Keyboard shortcut for "Show Desktop"
The F11 key shows the desktop by default.

If you would like to change it, you can in the keyboard preference pane under keyboard shortcuts, Mission Control. Feel free to make it Ctrl-Alt-D or Fn-D

Be aware that the function keys normally map to volume control / brightness, so you have to hold the fn key down to get F11 to be F11 instead of  (the best I can do for the volume down icon that coexists on the key labeled F11.)