Firstly, the names refer to their places in the congress during, I want to say 1905, but I could be wrong. Anyway, the Mensheviks split from the Bolsheviks. The name comes from the Russian word for minority, "menshestvo" versus the Bolsheviks' majority, or "bolshenstvo."
That said, the largest difference, in my opinion, is that the Mensheviks thought that they could work with the provisional government and the bolsheviks wanted to completely destroy the provisional government to create the communist state. The provisional government was a parliament that was set up by the revolutionaries upon the abdication of Nikolai II.
I wish I could help you more, but I've already consulted wikipedia to refresh my brain far more than I would like. Hope you find the information you're looking for!
Bachelor's Degree in Russian Studies, including class on 20th century Russian history