How 555 Timer Works?

555 Timer
The 555 monolithic timing circuit is a highly stable device, that can act either as a time delay generator (Monostable/one-shot), a Pulse-Width-Modulator (PWM), a RC Oscillator, or a Voltage
Controlled Oscillator (VCO).

In the time delay mode of operation, the time is precisely controlled by one external resistor and one external capacitor.

For a stable operation as an oscillator, the free running frequency and the duty cycle are both accurately controlled with two external resistors and one external capacitor.

The circuit may be triggered and reset on falling waveforms (trigger, Reset), and the output can source or sink up to 200mA.

APPLICATIONS
Precision Timing
Pulse Generation
Sequential Timing
Time delay Generation
Pulse Width Modulation


FEATURES
Turn-off time less than 2ms
Max. operating frequency greater than 500kHz
Timing from microseconds to hours
Operates in both astable and monostable modes
High output current
Adjustable duty cycle
TTL compatible
Temperature stability of 0.005% per °C

you can Visit the link for more details
http://courses.ncsu.edu:8020/ece480/common/htdocs/480_555.htm#description
http://courses.ncsu.edu:8020/ece480/common/htdocs/pdf/555AN.pdf