Vladimir Lenin (his real last name was Ulyanov) was the first leader of the Soviet Union. Lenin was a communist revolutionary who led the Russian communists in the overthrow of the previous governments (in 1917) and in its earliest days. He died in 1924.
Lenin is also important because he was the first person to put communist theory into practice in the real world. Lenin came up with a theory (known as Leninism) that applied Marxist theory to the conditions of Russia. It set out a blueprint for how to move a country towards a communist system. In this way, he took Marx's theories and figured out a way to apply them.