A quasar (or Quasi-Stellar Radio Source) occurs when gas near a supermassive black hole at the centre of a distant galaxy goes into the black hole (at very high speed), but electromagnetic forces cause it to swirl around above the hole and blast off into space in the form of huge jets of energy. When the gas gets close to the black hole, the gas heats up because of friction. Therefore, the gas glows very brightly, and this light is visible on the other side of the Universe. It is often brighter than the whole galaxy that the quasar is in. The first quasars were discovered with radio telescopes in the late 1950s and are still actively studied by astronomers today.
Astronomers now think that when a galaxy has a quasar, the quasar changes the galaxy. Gas and dust from the galaxy falls onto the quasar, and the bright quasar heats up gas in the galaxy. This stops new stars from forming in the galaxy, so many of the elliptical galaxies we see in the universe now may have once had a quasar in their centers. When the gas and dust stop falling onto the quasar and firing out, it stops being so bright and the black hole becomes very hard to see.
Galaxies with a quasar in process are often called active galaxies. Artist illustration