The Mensheviks was a political group in Russia which formed out of the Russian Social Democratic Labour Party.
The Mensheviks split from the Bolsheviks in 1903 was on the issue of how to undermine the Tsarist regime best. The Mensheviks wanted to reform the system while the Bolsheviks wanted to complete overthrow the system which they succeeded in doing in the Russian October Revolution of 1917.
Menshevik and Bolshevik are simply derived from Russian words for minority and majority. Yup the Bolsheviks went in 14 years from a splinter group of an illegal opposition party to running the whole shebang.