A neutron star is a celestial object composed of neutrons. It is produced in the final stages of a star's life during the collapse of the core, if the core's mass is between 1.4 and 3.0 solar masses it will be compressed under gravity under such intensity that the electrons will be driven into the protons to form neutrons, and the neutrons will be compacted such that the only reason that further collapse does not occur is simply because there is no space.
A neutron star will have a diametre between 10 and 15 kilometres and a density of 1017kg/m3. They possess very strong magnetic fields and release intense steams of radiation (mostly in the form of x-rays) from their magnetic poles as they spin. If the earth is within the line of these streams it will detect the star as pulses of radiation, and in this case the neutron star is known as a pulsar.