555 timer is the most standard IC circuit, so I am going to skip its working, fundamentals and come straight to the point. 555 timer is mostly used as an oscillator. The frequency of oscillations is controlled by a RC circuit. A capacitor and resistor of known value are taken to make a RC circuit of desirable time constant. Enough of theory lets get to the fun part ( if you want to know the real mechanism of 555 then follow this link) . Below is the schematic of the circuit.
Check out how this circuit looks when its working: