Communism vs. socialism vs. democratic socialism vs. social democracy

Six terms, one tight paragraph each. These definitions are shared verbatim across all four sister sites (democraticsocialism101.com, capitalism-101.com, and fascism-101.com); if one changes here, it changes everywhere. Most public arguments that use these words interchangeably are settled by reading this page.

Communism
In Marxist theory, the end-state, a stateless, classless, moneyless society with common ownership of the means of production. In common usage, the political systems run by Marxist-Leninist parties, which never reached and mostly stopped claiming to be near that end-state.[TODO-VERIFY]
Socialism
The broader family: social ownership of the means of production, achieved by many claimed routes (state, cooperative, municipal, worker control).[TODO-VERIFY]
Marxism-Leninism
The vanguard-party model actually implemented: single ruling party, central planning, "dictatorship of the proletariat" as an indefinitely extended stage.[TODO-VERIFY]
Democratic socialism
Social ownership pursued through multiparty electoral democracy with civil liberties intact.[TODO-VERIFY] See democraticsocialism101.com.
Social democracy
Capitalist market economy plus strong welfare state and labor protections (the Nordic model lives here, not under communism).[TODO-VERIFY] See democraticsocialism101.com and capitalism-101.com.
State capitalism
State-directed economy that preserves capital accumulation and wage labor; a term applied by critics to the USSR and to post-1978 China.[TODO-VERIFY]

The distinctions that do the most work

Communism is not a synonym for socialism: it is one branch of the family, defined by its end-state and, in practice, by the vanguard party (see M-02). Democratic socialism is separated from communism by multiparty democracy itself, not by degree (M-03). The Nordic countries are capitalist welfare states, which is social democracy, two rungs away (M-03). And a government doing things, from fire departments to Medicare, is not communism by any definition on this ladder (M-06, M-09).