What are the differences between red giants, white dwarfs and nebulas?
All of these bodies are related to each other in the evolution of stars. Sort of like the difference between a baby, a teen, middle aged adult and a senior!
Stars from out of nebulae, which are dispersed gases floating in enormous regions of space. Gravity starts to draw these gases, mostly Hydrogen and Helium, together and when enough mass collapses together, nuclear fusion takes over and the star lights up.
If the mass of the star is on the smallish side, it will eventually become a Red Giant later in its life - this is the future of our own sun in about 4-5 billion years. Near the end of a smallish star, it will again spit out gases and matter into another nebulae and eventually contract to a white dwarf star.
If the original mass of the star was large, it will eventually form a Red Supergiant star. This kind of star will explode as a supernova and then become either a neutron star or a black hole. These kinds of supernova events probably produced most of the elements in the chemistry periodic table.