The Mensheviks were a faction of the Russian revolutionary movement that emerged in 1903 from a dispute between Vladimir Lenin and Julius Martov, both members of the Russian Social Democratic Labour Party. At the second congress of the RSDLP, Lenin argued for a small party of professional revolutionaries with a large fringe of non-party sympathizers and supporters. Martov disagreed believing it was better to have a large party of activists. A majority of party members agreed with Martov and formed the Mensheviks, while Lenin's faction became known as the Bolsheviks. Although a majority of rank and file party members agreed with Martov, they formed a minority among the party leadership, and hence Menshevik is a Russian word meaning "minority" while Bolshevik means majority.
The Mensheviks played a leading role in the 1905 Revolution and were particularly active in the Soviets and the emerging trade union movement.
The split between the two branches were long standing, and had to do with issues both pragmatic and based in history (the failed revolution of 1905) and theoretical (issues of class leadership, class alliances, and bourgeois democracy). The Bolsheviks felt that the working class should lead it in an alliance with the peasantry, where the Party acts as extreme revolutionary opposition, while the Menshevik vision was one of a bourgeois liberal revolution. The Menshevik vision was more along that of Western Democracies, and a gradualist approach to socialism.
Many Mensheviks left the party after the defeat of 1905 and joined more legal opposition organisations. After a while, Lenin's patience wore out with their compromising and in 1908 called the Mensheviks "liquidationists" and this eventually led to the Bolsheviks forming their own party in 1912. The Mensheviks further split in 1914 with the advent of the war. Most Mensheviks opposed the war, but a vocal right-wing minority supported it in terms of "national defence". After the revolution of 1917, most Mensheviks supported the war effort under the slogan "defence of the revolution". The left wing of the party, led by Martov, was strongly critical of this position, and was completely aghast at the party's decision to join a bourgeois socialist coalition government.
Lenin's more radical positions grew in popularity during the First World War as anger mounted against the czarist regime, and a number of leading Mensheviks such as Leon Trotsky and Alexandra Kollontai joined the Bolsheviks.
This split in the party crippled their popularity, and they received less than 3% of the vote compared to the Bolshevik's 20%. The right wing of the Menshevik party supported right-wing actions against the Bolsheviks, while the left wing, the majority of the Mensheviks at that point, supported the Left in the ensuing civil war.
The Bolsheviks won the civil war and subjected the Mensheviks to great repression. Menshevism were finally made illegal after the Kronstadt Revolution of 1921. A number of Prominent Mensheviks emigrated thereafter. Martov went to Germany, where he died in 1923.