Until 1903 the Bolsheviks and the Mensheviks were a single party called the Social democrats. They wanted a communist Russia, following the ideas of Marxism. However with in the party of the social democrats there were two men, Lenin and Plekhanov whom of which had two completely different ideas on how the party was going to be successful in achieving the revolution, causing a split in the social democrats.
Both parties’ were aiming for the same thing but in different ways. The Mensheviks thought that the get revolution the best thing to do was to wait for the bourgeois stage had to happen first. But Lenin thought this was ridiculous and he proclaimed that both the proletarian and bourgeois stages could me merged into one revolution. Another issue the parties disagreed on was the party and its size. Lenin said the smaller the better as it would be more elite and easier to control and mange so they would get the job done faster as the party were smaller so less chain of command, but the Mensheviks thought that the party need to be big so they looks strong but Lenin thought this was a risky choice because of the secret police the more people in a party the more likely information is to be spread.
Plekhanov thought that everyone in his party should have a say or a vote when it came to making decisions but Lenin again thought this was ridiculous has he believed that would take too long, Lenin thought that democratic centralism was the way, this meant members were obedient and leaders gave authority so Lenin gave orders, he was in charge, Plekhanov and Martov seen this like Lenin was a dictator. The Mensheviks wanted to improve working conditions and get pay rises, support the trade unions but Lenin thought that this was a silly idea as they want to make the Russians lives worse so they want the Bolsheviks to take control as it will help them.
In conclusion, the split happened in 1903