What is the difference between stopping and destroying an instance?

Stopping an instance is the virtual equivalent to powering the server off.  The virtual machine still is there; it is just "down" and unavailable until it is restarted.  You stop a machine by clicking on the red octagon on the far right of  your browser.

Stopping

Once a machine is stopped, the buttons on the browser change.  The octagon disappears and is replaced by a green circle.  Clicking the Restartbutton will start the machine up again.

Destroying a virtual machine is, however, permanent and irreversible.  You destroy machines you no longer need or want.  Be very certain that is what you do when you click the Terminatebutton  You will be asked to confirm that this is what you indeed intend.  ONCE YOU HAVE DESTROYED A VM, IT CANNOT BE RECOVERED.