Communism (Russian: коммунизм) is a social-economic system based on a society that is without classes and without sovereignty. It is a branch of socialism that dates from the Industrial and French Revolutions. It is based also on common ownership of the means of production. Communism is the opposite of Capitalism, which guarantees a lower class and upper class.
Communism can be divided into two broad categories: Marxist communism and non-Marxist communism. Although the dominant forms of communism are variants of Marxism, communism and Marxism are not same. Marxism is a branch within communism, not all communists are Marxists. There are non-Marxists version of communism like anarchist communism and Christian communism.
Marxism-Leninism is a variation of communism theorized by Lenin. It is the most common form of Communism used by Communist parties around the world. The Former Soviet Union was unable to create egalitarian society, instead it was actually a party dictatorship and state capitalist country.
Karl Marx and Frederick Engels are the two people who are accredited with inventing modern communism. Marx argued that capitalism caused the existence of two opposing classes of people and their interests where opposing.
Communists disagree with Social Democrats in their philosophy, Communists maintain that the current system can never be changed to improve the favor of workers. They also disagree with Anarchists because Communist philosophy maintains that dismantlement must come gradually.
Marx hypothesized the "dictatorship of the proletariat" will eventually result in a classless society. Anarchist theorist Bakunin and Marx had disagreement over this. Bakunin predicted "dictatorship of the proletariat" is impossible because the proletariat cannot rule over others, and to benefit the proletariat, the state should be abolished, but Marx did not pay attention to his suggestion. Soviet Union was never classless, it had a bureaucracy who enjoyed greater freedom than the general population called "Nomenclatura".