The keyboard shortcut for "Minimize All" is Win+M and The keyboard shortcut for "Show Desktop" is Win+D. How are they different?
"Minimize All" is easier to describe. It minimizes all the windows that support the "Minimize" command. You can minimize a window by selecting "Minimize" from its System menu, or by clicking thebutton in the title bar. So "Minimize All" is effectively the same as going to each window that is open and clicking the Minimize button. If there is a window that doesn't have a Minimize button, then it is left alone.
"Show Desktop" takes "Minimize All" one step further. After minimizing all the windows that can be minimized, it then takes the desktop and "raises" it to the top of the window stack so that no other windows cover it. (Well, okay, topmost windows continue to cover it.)
So "Show Desktop" manages to get a few more windows out of your way than "Minimize All".
Note, however, that when you return the desktop to its normal state (either by selecting "Show Open Windows" or just by switching to another window), all the un-minimizeable windows come back because the desktop has "lowered" itself back to the bottom of the window stack.