A pulsar is a neutron star that emits beams of radiation that sweep through Earth's line of sight. Like a black hole, it is an endpoint to stellar evolution. The "pulses" of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star's rotation axis and its magnetic axis. Pulsars seem to pulse from our perspective because the rotation of the neutron star causes the beam of radiation generated within the magnetic field to sweep in and out of our line of sight with a regular period, somewhat like the beam of light from a lighthouse. The stream of light is, in reality, continuous, but to a distant observer, it seems to wink on and off at regular intervals.
Pulsars are the original gamma-ray astronomy point sources. A few years after the discovery of pulsars by radio astronomers, the Crab and Vela pulsars were detected at gamma-ray energies. Pulsars accelerate particles to tremendous energies in their magnetospheres. These particles are ultimately responsible for the gamma-ray emission seen from pulsars.
In this video, gamma rays are shown in magenta. Data from NASA's Fermi Gamma-ray Space Telescope indicate that most of the gamma rays emitted by a pulsar arise from far above the pulsar's surface.