How do quasars and active galaxies differ from normal galaxies?
Quasars and active galaxies have material falling into a supermassive black hole.
Most galaxies have a supermassive black hole at their centres. Most galaxies have little material falling into the black hole.
Young galaxies and galaxies which have collided with others have an abundance of dust and gas near to the central black hole. This material forms an accretion disk as the material falls into the black hole.
As more and more material falls into the accretion disk is gets superheated due to friction and gravitational effects. This then generates a lot of energy which is emitted as electromagnetic radiation. This energy output makes the galaxy a quasar or active galaxy.
Once the supply of material falling into the accretion disk is exhausted, the galaxy becomes a regular non-active galaxy.