Leninism is not to be confused with Marxism, but it often is. Marxism is a theory of history with little practical application, whereas Leninism is a blueprint for political organization in the real world. Marx predicted incorrectly that society would move through a series of stages automatically over time, with socialism finally emerging in exhausted industrial societies. Lenin was impatient and wanted to make a revolution in agrarian, non-industrialized Russia, and so he made some innovations.
The central concept of Leninism is the vanguard party, a group of professional revolutionaries who focus on organizing the proletariat and spreading propaganda to create what they called revolutionary consciousness. Marx said the workers would unite for revolution themselves; Lenin said that the workers had to be taught to want revolution. Hence the vanguard party had to force socialism into existence, rather than waiting for it to appear automatically.
Marx theorized that the dictatorship of the proletariat would be the rule of a whole social class arising spontaneously from below; Lenin implemented a literal dictatorship of the vanguard party from above. Hence, while much has been made of the differences between Leninism and Stalinism, the development of something similar to Stalinism is an integral risk of Leninism: how else is a small band of conspirators to impose wholesale social and economic change on a society but through violence? As a programme for practical action, Leninism makes moral judgements about social classes in a way Marx did not: Marx had praise for the bourgeoisie as a necessary phase in society's evolution, whereas Lenin's doctrine required their forced extermination. Combined with the absolute loyalty that a party which saw itself as embodying history demanded, it was a short step indeed to kulaks and gulags and terror-famines.