Quasars are intense sources of X-rays as well as visible light. They are the most powerful type of X-ray source yet discovered. Some quasars are so bright that they can be seen at a distance of 12 billion light years.
The power of a quasar depends on the mass of its central supermassive black hole and the rate at which it swallows matter. Almost all galaxies, including our own, are thought to contain supermassive black holes in their centers. Quasars represent extreme cases where large quantities of gas are pouring into the black hole so rapidly that the energy output is a thousand times greater than the galaxy itself. A galaxy with a somewhat less active supermassive black hole is called an Active Galaxy and its black hole is called an 'Active Galactic Nucleus' or AGN. Our Milky Way Galaxy and our neighbor, the Andromeda Galaxy, are examples of normal galaxies, where the supermassive black hole has very little gas to capture.
'We are all in the gutter, but some of us are looking at the stars.'