As Soviet Snake said Marxist-Leninism is the development of Marxist theory and applied to Russian conditions. What sets Lenin’s theories apart from Marx’s as in what distinguishes marxist- leninism from marxism? One of the core parts of it is that communism must be protected by a vangaurd party of the people and upheld by a state, and that socialism could only be achieved as the natural evolution of that and the no longer of any need for the vanguard to govern as in, the people truly rule themselves. The vangaurd party must be democratic centralist and can only receive power if it is handed to them by the will of the workers. (And to distinguish Marxist-Leninism from Troskyism: Trotsky tried to develop a theory of Marxism which was to implement this theory at all costs/permenant revolution, to take power regardless of the consciousness of the working class, this aligned his politics with fascists at the time.)

Again this is a basic understanding of the ideology and history of practice, the development of this theory arose from the will of the people with Lenin & Stalin as true guides of the revolution, which was happening in Russia at the time regardless because of the Tsar monarchy and the dog shit working conditions of the Russian people. Critics tend to see Lenin as an opportunist because of this but much like Marx was able to succinctly define the nature of class and material production in his theories, Lenin truly understood this in practice and was able to implement it according to the will of the people.