The 555 timer is probably the most famous chip ever produced. It has been used in countless applications and is very cheap to produce.
The chip contains two comparators and a flip-flop.
The comparators will compare voltages running through the chip, while the flip-flop will alternate the output of the chip, giving either a low or high voltage (pin 3).
Notice the little "dot" at the top of the chip? This is here to help you turn the chip the right way when inserting it on a breadboard. It is always at the top (like indicated above), with pin 1 on the top-left. You then count the pins anti-clockwise.