Top-left: Monostable mode (one shot).
Bottom-left: Bistable mode (aka flip-flop).
Right: Astable mode (most common).
See also http://www.circuitbasics.com/555-timer-basics-monostable-mode/
Note: Pin 5 control voltage capacitor optional. It filters any noise that may be on the VCC power and that would otherwise cause erratic operation.
