A blazar is a quasar whose relativistic jet points almost directly toward Earth.
Quasars are extremely luminous active galactic nuclei powered by matter falling toward a supermassive black hole. Some launch narrow jets of particles moving close to the speed of light. When one of those jets is aimed near our line of sight, the object is classified as a blazar.
The alignment produces relativistic beaming: radiation is concentrated toward Earth, making the source appear exceptionally bright and highly variable. Blazars can change brightness over hours or even minutes, depending on the observing wavelength and emitting region.
A blazar is not the same thing as a pulsar or magnetar. Pulsars and magnetars are rotating neutron stars, while blazars are distant galaxies with active central black holes. The category includes two major types: BL Lacertae objects and flat-spectrum radio quasars.