Pulsars :

A pulsar is a highly magnetized, rotating neutron star that emits a beam of electro-magnetic radiation. This radiation can only be observed when the beam of emission is pointed towards the earth.
A neutron star is pretty dense and has short-regular rotational periods.A pulsar gets it's name because it emits energy in the form of pulses and is short for "Pulsating star".

When and by who'm was the first pulsar discovered?

The  first pulsar was observed on November 28, 1967 by Jocelyn Bell Burnell and Antony Hewish.They observed pulses seperated by 1.33 seconds, originated from the same location on the sky.