I’m gonna leave out utopian communism because it’s not what we’re talking about and isn’t really relevant.

Communists are working toward a classless, stateless society. Viewing the world through the lens of class struggle, they see the state as a tool of class repression and seek to use it to get to that stateless classless society. Its important here to recognize that communists want to use the tools of capitalism to develop the productive forces on the way.

Socialism is worker control of the means of production.

Social democracy is a set of policies enacted by bourgeois societies to keep people from revolting.