What is the the difference between a quasar and a galaxy?

Supermassive black holes, like the one in our Milky Way, are located at the center of most massive galaxies.

Large amounts of gas and dust surround the centers of some galaxies, especially young galaxies. This material forms an accretion disc, which is spiraling into the supermassive black hole. As more material falls into the accretion disc, it heats up due to gravity and fiction to the point where it emits enormous amounts of energy; this is known as a quasar.

Only as long as there is enough material falling into the accretion disc can quasars exist; once this material runs out, the active galaxy transforms into a regular galaxy.

Observed quasars are young galaxies as observed in the early universe because they are billions of light years away.

Seyfert galaxies, which function similarly to quasars but don't produce as much energy, are a related phenomenon. Any galaxy has the potential to explode as a quasar if enough material is present around its central supermassive black hole; this usually occurs in young galaxies and during galaxy mergers.

Galaxies are massive systems of stars, gas, and dust held together by gravity. A quasar is an exceptionally bright and energetic active galactic nucleus, frequently found at the center of a galaxy.