Leon Trotsky was one of the leading theorists of the Bolshevik Party, though early in the 1900's -- up until mid-1917 -- he sided with the Mensheviks who advocated for a more democratic approach to socialism.
Trotsky was arrested several times for revolutionary activities throughout his life but managed each time to escape. While in exile in Vienna, he founded and wrote for Pravda, a Russian newspaper for the "workers."
When the revolution broke out in 1917, Trotsky returned to Petrograd to help overthrow the Provisionary Government and was welcomed into the Bolshevik party in August. Trotsky served as the military leader of the revolution in Petrograd at the time Alexander Kerensky was trying to assert control of the Provisional Government.
With the successful uprising in November, Trotsky was second only to Vladimir Lenin in the Bolshevik Party. He would go on to rebuild the Red Army at the end of the Russian Civil War.
When Lenin's health declined, Joseph Stalin and others banded together against Trotsky, who had been an obvious successor. Trotsky was eventually exiled from Russia and was murdered in 1940.