The primary difference between Marxism-Leninism and Maoism is who the ideologies view as the most important class for the revolution.

Orthodox Marxism says that the proletariat, the working class, such as the factory workers or the farmers, will overthrow the capitalist working class and establish a socialist state.

Marxism-Leninism says that the proletariat must be helped with a vanguard party and the vanguard party must take over after the revolution to carry out the policy change in order to make a socialist state functioning.

Maoism, however, diverges from this and says the primary revolutionary class are the peasantry and training all men in guerilla warfare for a people's war as part of class struggle (Something akin to Vietnam, even though Vietnam was Soviet leaning during the split).

Those are at least the key differences in the theory.

As for the history, they were distinctly difference before the Sino-Soviet Split, however the split made them even more distinct. While the Soviet Union was in the process of "destalinization", the People's Republic of China and Albania (who was Chinese leaning during the split), both admired the policies of Stalin. Enver Hoxha (leader of Communist Albania), even distinctly believed that the only true communist states were Albania, the PRC, and the Soviet Union before 1953.