555 Timer, Operation.

Astable Operation Oscillator	The output at pin 3 produces a square wave output, oscillating at a frequency determined by 2 resistors and a capacitor. Astable Operation (Oscilator)
Monostable Operation One-Shot	The output at pin 3 produces a pulse every time a push button is operated, the duration of the pulse is determined by a resistor and a capacitor. Monostable Operation (One-Shot)
Monostable Operation Delay On	The output at pin 3 goes high after a set delay, the duration of the delay is determined by a resistor and a capacitor. Monostable Operation (Delay On)