A pulsar is a highly magnetized, rotating neutron star. It emits a beam of electromagnetic radiation that pulses in a regular, precise duration. Neutron stars are very dense, and have short, regular rotational periods that are connnected with the extremely strong magnetic fields which spin with them. Durations between pulses can be from sub-milliseconds to seconds.
Observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of gravity waves. The first exoplanet was discovered around a pulsar.
Several hundred pulsars are now known, and can be observed because their electromagnetic radiation points to Earth where it can be analyzed as you would a constantly turning beam from a lighthouse.