A quasar is the blazing core of a distant galaxy, powered by a supermassive black hole actively pulling in surrounding gas and dust. As that material spirals inward, it heats up and radiates enormous amounts of energy, often outshining every star in its host galaxy combined.
The name is short for quasi stellar radio source, coined because early quasars were first picked up as compact radio emitting points that looked deceptively like ordinary stars through a telescope, only far more powerful and far more distant.
Because quasars are so bright, they can be seen across billions of light years, giving astronomers a rare window into what galaxies looked like when the universe was young. More detail is available from NASA’s Imagine the Universe.