This is perhaps the most useful keyboard shortcut you can learn, and it’s very simple.
Command-Tab = Switch to the next application
When you want to move to another application that you have already opened, instead of moving the mouse cursor down to the app’s icon on the tool bar, just hold down the ⌘ key and tap the tab key to bring up the panel displaying all open apps. While holding down the ⌘ key keep tapping that tab key until the desired application is selected and then let go of both keys.
Command-Shift-Tab = Switch to the previous application
If you accidentally go past the app that you were looking for while tapping on the tab key then hold down the shift key (while still holding the ⌘ key down) and then tap the tab key again and the selector backwards to the previous application.
– Chapter 34, Tao Te Ching.