Almost all normal galaxies have a supermassive black hole at the center, including ours. But we can’t actually see the black holes themselves. They’re so dense and their gravitational field is so strong that nothing can escape them, not even light. So we observe them instead by seeing how the matter around them behaves.
All of the ‘stuff’ in these galaxies is very slowly falling in toward the black hole, causing the galaxy to spiral and form into a disk shape. Some of these galaxies are special: the supermassive black hole is consuming more than the normal amount of the galaxy around it, and faster--it’s greedy. These are called active galactic nuclei, or AGN for short.
But there’s a theory that posits something a little radical, called AGN unification theory. First proposed back in 1993, it claims that essentially, all AGN are the same, but they just look different to us based on their orientation in space.