How are quasars, seyfert galaxies, radio galaxies, and normal galactic nuclei related?
Quasar, Seyfert galaxies and radio galaxies are all associated with active galactic nuclei
It is thought that most large galaxies have a supermassive black hole at their centre.
Regular galaxies have relatively little material in the vicinity of the black hole and hence don't have active nuclei.
If there is an abundance of dust and gas in the vicinity of a galactic supermassive black hole, then the material falls into the black hole forming an accretion disc. Material falling into the accretion disc gets superheated by friction and gravitational effects until it emits huge amounts of radiation. This is the power source of an active galactic nucleus.
Quasars are very powerful and very distance active galactic nuclei. They are billions of light years away. This makes them young galaxies as they were billions of years ago.
Seyfert galaxies are visible galaxies with active nuclei. They have unusually bright core regions.
Radio galaxies have active nuclei which emit mainly in the X Ray spectrum. These are normally associated with elliptical galaxies which have particularly large supermassive black holes.