The RSDLP was not the first Russian Marxist group; the Emancipation of Labour group was formed in 1883. The RSDLP was created to oppose narodnichestvo, revolutionary populism, which was later represented by the Socialist-Revolutionary Party (SRs). The RSDLP program was based on the theories of Karl Marx and Friedrich Engels - that, despite Russia's agrarian nature, the true revolutionary potential lay with the industrial working class. The RSDLP was illegal for most of its existence; at the end of the first party congress in March 1898, all nine delegates were arrested by the Imperial Russian Police. At this time there were 3 million Russian industrial workers, just 3% of the population.
Before the Second Congress, a young intellectual named Vladimir Ilyich Ulyanov joined the party, better known by his pseudonym—Vladimir Lenin. In 1902 he had published What is to be Done?, outlining his view of the party's task and methodology—to form "the vanguard of the proletariat." He advocated a disciplined, centralized party of committed activists who sought to fuse the underground struggle for political freedom with the class struggle of the proletariat.
In 1903, the Second Congress of the party met in exile in Brussels to attempt to create a united force. However, after unprecedented attention from the Belgian authorities the congress moved to London, meeting on August 11 in a chapel in Tottenham Court Road. At the congress, the party split into two irreconcilable factions on November 17: the Bolsheviks (derived from "Bolshinstvo"—Russian for "majority"), headed by Lenin, and the Mensheviks (from "Menshinstvo"—Russian for "minority"), headed by Julius Martov. Confusingly, the Mensheviks were actually the larger faction, however the names Menshevik and Bolshevik were taken from a vote held at the 1903 party congress for the editorial board of the party newspaper, Iskra ("Spark"), with the Bolsheviks being the majority and the Mensheviks being the minority. These were the names used by the factions for the rest of the party congress and these are the names retained after the split at the 1903 congress. Lenin's faction later ended up in the minority and remained smaller than the Mensheviks until the Russian Revolution of 1917.
A central issue at the congress was the question of the definition of party membership. Martov proposed the formulation "A member of the Russian Social-Democratic Labour Party is one who accepts the Party’s programme, supports the Party financially, and renders it regular personal assistance under the direction of one of its organizations." Lenin, on the other hand, proposed a more strict definition: “A member of the Russian Social-Democratic Labour Party is one who accepts its programme and who supports the Party both financially and by personal participation in one of the Party organizations." Martov won the vote, and the bolsheviks accepted it as part of the adopted organizational rules.
Despite a number of attempts at reunification, the split proved permanent. As time passed, more ideological differences emerged. According to many historians, the Bolsheviks pushed for an almost immediate "proletarian" revolution, while the Mensheviks believed that Russia was still at too early a stage in history for an immediate working-class revolution. The two warring factions both agreed that the coming revolution would primarily be "bourgeois democratic" in its character. But while the mensheviks viewed the liberals as the main ally, the bolsheviks opted for an alliance with the peasantry as the only way to carry out a popular revolution while defending the interests of the working class. Essentially, the difference was that the bolsheviks considered that in Russia, the tasks of the bourgeois democratic revolution would have to be carried out without the participation of the bourgeoisie.