Marxism–Leninism

Marxism–Leninism was practiced by the Soviet Union (USSR) after the Bolshevik Revolution. It was also practiced by the Stalinists in the USSR. 
The purpose of Marxism–Leninism is to turn a capitalist state into a socialist state. This is done by a revolution by the proletariat to overthrow the old government. The new government then creates a "dictatorship of the proletariat". This new government leads the country based on democratic centralism.
The communist party is supposed to lead society from capitalism to socialism. According to ML, socialism was more "advanced" than capitalism, but communism was more "advanced" than socialism. Indeed, the ultimate goal is to achieve a communist society, one with no class or state. ML just provides a first step: from capitalism to socialism.
Today, the only remaining countries who are adherents of Marxism-Leninism are: China, Cuba, Vietnam, and Laos, with North Korea moving to its own political philosophy, Juche, though the ideology was also influenced by Marxism-Leninism.
Lenin introduced the NEP in 1921 because Russia was in a state of collapse. 
Beliefs.
Marxism–Leninism is an extension of Leninism. It developed as a form of Marxism with adaptations from the works and theories of Vladimir Lenin and Joseph Stalin. Mao Zedong has also made contributions to Marxism–Leninism through Maoism.