Marxism-Leninism was the official ideology of the USSR. It developed after Lenin's death when competing factions within the Soviet leadership attempted to establish their legitimacy as Lenin's political successors. Soviet leaders such as Leon Trotsky, Grigory Zinoviev, and Joseph Stalin wrote works developing the concept of a specifically Leninist ideology. Joseph Stalin coined the term to describe an ideology which considers Lenin's political thought to be a necessary development of Marxism, and made the term popular through The History of the Communist Party of the Soviet Union (Bolsheviks) (1938).
(factional struggle between Trotsky and Stalin/Zinoviev/Kamenev/Bukharin)
(consolidation of Marxism-Leninism)
A Marxist-Leninist party is organized according to democratic centralism. This means that first the party democratically decides something, and then all members are required to follow that decision and not work against it. If members still disagree with the decision, they are expected to request to discuss the issue again, not to form a faction or another party.
Socialism in one country
(socialism in one country)
Political economy, crises and revolution
(political economy, crises, & revolution)
(the lower phase of communism aka socialism)
Marxist critics argue that Marxism-Leninism works from the assumption that the Soviet Union was a socialist society and use this as reference point to navigate both political theory and practice. The consequence of this being that Marxism-Leninism is reconciled with bourgeois concepts such as the nation-state and nationalism. The most common criticism of Marxism-Leninism from Trotskyists is that rather than seeking to spread the socialist revolution throughout the world through direct revolutionary action, hence pursuing "Proletarian Internationalism," Stalin instead only assisted existing socialist revolutions, believing that the Soviet Union was the only country in the world capable of achieving socialism due to the success of its own revolution, and actually enacted a foreign policy of "peaceful coexistence" with the Western capitalist powers.  Stalin named this theory Socialism in one country.
(Trotskyism aka Bolshevik-Leninism)
(Trotsky's criticism of Marxism-Leninism)
The History of the Communist Party of the Soviet Union (Bolsheviks) (1938)