The Bolsheviks were a faction of the Marxist Russian Social Democratic Labour Party (RSDLP). They were considered professional revolutionaries and were founded by Vladimir Lenin.
They seized power in Russia in 1917 October Revolution and in turn founded the Soviet Union that we know today. Their beliefs was often called Bolshevism.

Their symbol is shown below, the sickle/hammer flag. This is sometimes a symbol of general communism for people around the world now.