The 555 integrated circuit is commonly called the "555 timer" but that is only one of it's uses. The chip can be used both as a timer and as an oscillator.
WHAT DOES THAT MEAN?
Well i'm glad you asked. An Oscillator is something that produces and constant electrical wave, like a sine or square wave. The timer will pulse at a specific interval depending on the components attached.
The 555 chip has 3 main modes:
Monostable Mode for "One Shot" pulse generation:
Astable Mode for putting out a continuous stream of pulses. (aka timer):
Bistable mode for acting like a Schmitt trigger.
The Pin out of the 555 timer is as follows:
Pin 1: Ground
Pin 2: Triggering the timer, connecting this to ground starts the chip up.
Pin 3: Output, stays at whatever Vcc is
Pin 4: Forces pin 3 to low if grounded
Pin 5: Used to adjust trigger threshold.
Pin 6: Threshold that ends timer when 2/3 of Vcc is reached.
Pin 7: Discharge, connects to ground when output is low
Pin 8: Power, denoted in most contexts as Vcc
Images above are borrowed from: http://clarkson-uk.com/555-timer/