What is the difference between a nebula, quasar and red giant?

A nebula is a cloud of gas that surrounds a star that has exploded in a supernova. So it is essentially the remnants of a star that has died. A quasar is an extremely active black hole in the centre of a galaxy that spews out EM radiation. A red giant is a star that has reached the end of the main phase the envelope of the star is inflated which is what makes them giant.