How does a quasar differ from a galaxy?
Quasars are different from galaxies in the sense that they are most likely black holes, whereas galaxies are large collections of stars.
The truth is that very little is known about quasars. Most likely, quasars are massive black holes, that are surrounded by accretion disks. Accretion disks are flat disks of gas that usually spiral around black holes. However, some believe them to be super luminous galaxies. What is known about quasars is that they are extremely distant celestial bodies that emit an unbelievable amount of energy at high speeds.
Galaxies are huge systems of stars held together by gravitational fields and separated from other groups of stars by large amounts of space.