What do active galaxies differ from normal galaxies?

Galaxies that emit much more radiation (different types of radiation) than a typical galaxy. This actually happens in the galaxy's center, so it's more accurate to talk about an active galactic nucleus. The active galactic nucleus is believed to be caused by a black hole. From Wikipedia: " The radiation from AGN is believed to be a result of accretion of mass by a supermassive black hole at the centre of its host galaxy."