"Show desktop" keyboard shortcut in Mac OS X
On older versions of OS X, it's one of the Exposé keys. Starting with OS X 10.7, Exposé has been replaced by Mission Control.
The default is F11. On a MacBook you will have to press fnF11, as the keys are used for controlling volume by default.
Note that F11 will not work if you have a full screen window on screen (like the browser in full-screen mode), you will have to switch to the Desktop mode first (using F3, for example) before using F11.
If that's not the right one, look in your System Preferences to see which one it is.
If F11 by default decreases volume for you, you need to instead press FnF11. You can change that setting in the Keyboard System Preferences: Use all F1, F2, etc. keys as standard function keys . After enabling that, F11 will show desktop, and FnF11 will decrease volume.
Some people have mentioned that ⌘F3 also works to show the desktop.
If it's a new MacBook Pro, you can also use a three-finger spread to show the desktop. Check out Trackpad in the System Preferences for a demonstration of that:
Also, in the Exposé / Mission Control System Preferences, you can set up an Active Screen Corner where you can send the mouse to to show the desktop.
What others are referring to as CmdH refers to the ⌘H on your keyboard. That hides the current application and shows whatever's behind it, which may or may not be the desktop depending on what other applications are running. Switch to the Finder, close any Finder windows, and ⌥⌘H will hide all other applications, revealing the desktop. This would not show the edges of other windows.
Use ⌘F3 on Mavericks and newer Macbook Pros.
This doesn't require use of the Fn key. It's similar to ⇧⌘F3, but is instantaneous because it shows no animation.