Super-massive black holes are known to dwell at the center of galaxies. When accreting matter they are observed from Earth as active galactic nuclei (AGNs). In a minority of AGNs the accretion of matter onto the black-hole is associated with the ejection of a relativistic jet of plasma along the polar axis.
When the black-hole's jet points right in the direction of the Earth, relativistic effects boost the emission and make these objects among the brightest in the Universe. This peculiar AGNs are called blazars.