Joseph Stalin (1879-1953) was a revolutionary in his teens and until after the Russian Revolution of 1917. In the early 1920s he became the key leader of that section of the Bolshevik Party who, under pressure of isolation, exhaustion, and the extreme poverty of Russia, were abandoning their socialist ideals and joining up with the state bureaucrats inherited from the old regime.
The working class had been dispersed and battered by the long civil war (1918-21) against counter-revolutionaries backed by forces from no fewer than 14 other states. Stalin's faction defeated the loyal revolutionaries led by Trotsky, and from the late 1920s carried through a full-scale counter-revolution, transforming what had been a brave though flawed workers' state into a dictatorship more oppressive for the working class than any the world had ever seen before.
In his lifetime, Stalin was a hero for many workers in the official Communist Parties who did not know, or chose not to know, the realities of Russia under Stalin's rule, and took heart from its impressive claims for industrial growth. In 1956 Stalin was denounced by his successors in Russia's government, and gradually, after that, "Stalinism" came to be seen as something to be opposed by anyone with left-wing or democratic ideas.
Those successors also introduced some reforms in Russia, easing the police-state terror. However, they maintained the essentials of "Stalinism" - a state with a single bureaucratic "party" hierarchy controlling both industry and all the supposed "mass organisations", with no freedom of autonomous trade-union or other organisation for the working class - for many decades.
From 1985, in a drive to revitalise Russia's by-then stagnant economy, Mikhail Gorbachev tried to reform the system in a limited way, from above. In the event those attempts provoked, in 1989-91, the collapse in both Eastern Europe and Russia of a system which had long been rotting from the inside. Stalinist regimes exist today in North Korea, Cuba and - although it has overseen a great development of capitalism - China.