What is a Pulsar
A pulsar is a highly magnetrized rotating neutron star. They appear to pulse because they have jets of particles moving almost at the speed of light coming from their magnetic poles. These jets produce powerful beams of light. The magnetic pole is slightly misaligned from the rotational axis line, like the magnetic pole on Earth is not in line with our "true north". Picture holding a flashlight in your hand and rotating your hand around. The beam from the flashlight will appear to pulse as it heads towards you and then it stops as it swings away. We observe the beams of light from pulsars only when they are pointing towards Earth and then they appear to go off as the pulsar's magnetic pole is facing away from Earth. This is called the lighthouse effect. The period of pulses range from 1.4 milliseconds to 8.5 seconds.
Shown below is an artist's concept of a pulsar surrounded by material blown away by the supernova explosion.
Image from http://www.nasaimages.org