Spackle is used more as a filler for nail holes and small imperfections on walls. It dries relatively quickly & is used as a quick-fill product before painting. Joint compound is a gypsum-based product used to fill drywall joints. Most versions of it dry slowly. Occasionally the quick-setting versions can be used much like spackle, but, more often than not, JC is used to finish drywall. I'm not a chemist so I can't tell you the differences in product makeup.