Makey Makeys work just like a USB keyboard or mouse, sending keyboard (WASD, spacebar, etc) and mouse (left click, right click, etc) signals to your computer.
Makey Makey Classic works through opening and closing circuits, just like any other button. Instead of the circuit being closed underneath your keyboard, the circuit is closed through the conductive objects you connect with alligator clips like your hand or your lunch or some tinfoil. When the circuit is closed, the Makey Makey sends a command to your computer, just like when a button is pressed on a keyboard.
Makey Makey GO works through capacitance, like a button on a touchscreen. When you connect something to the Makey Makey GO, the GO senses the capacity of that object to hold electricity. When you touch the connected object, that capacity suddenly expands to include your whole body! And the Makey Makey GO senses the difference and sends a command to your computer.