Stars of relatively little mass will undergo violent convulsions as they become unstable at the end of their lives, causing large amounts of what mass they have to slough off into space as the red giant collapses. The mass that they slough off forms a shell around the star, known as a planetary nebula. The star meanwhile becomes a white dwarf.
Stars of relatively large mass will also undergo these convulsions in which they lose mass, but eventually, they come to a threshold when their fuel runs out: if they have lost enough mass, down to about (I think) 1.4 solar masses, they collapse into white dwarfs.
However, if they have more than that threshold, then the core collapses and becomes a neutron star or a black hole if it is really massive, and the energy released in that collapse blows the rest of the star apart, causing a supernova.
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