The Bolshevik Party formed in 1903 after splitting from the Russian Social Democratic Labour Party. The RSDLP split into two factions, the Bolsheviks and the Mensheviks. The Bolshevik Party was interested in total revolution of the Tsarist regime, while the Mensheviks were interested in reform.
The Bolshevik Party officially expelled the Menshevik faction in 1912. It was at this stage that it became the Communist Party of the Soviet Union. Both the original Bolsheviks and the Communist Party were headed by Vladimir Lenin.