A pulsar is simply a rapidly spinning neutron star. When a super giant star collapses to form a neutron star, the neutron star begins to spin faster due to the conservation of angular momentum (for more on this, see Why Do They Spin?). Pulsars are covered with an iron shell, and have extremely packed neutrons inside. Most of the mass and magnetic field of the pre-existing star ends up trapped and condensed inside the neutron star. This powerful magnetic field causes the neutron star to emit powerful radio waves from its 2 magnetic poles. Every time these poles pass by the earth, they disturb radio frequencies in a way that seems as if they are blinking.