Old Believers

In the Russian Orthodox church history, the Old Believers (, read "starovery" or "staroobryadtsy") became separated after 1666-1667. They split from the hierarchy of the Russian Orthodox Church as a protest against church reforms introduced by Patriarch Nikon. Old Believers continue liturgical practices which the Russian Orthodox Church followed before the reform. 
Russian-speakers refer to the schism itself as "raskol" (раскол). This word comes from a Russian word "расколоться", that means "to cleave apart").
Background.
Patriarch Nikon (1605-1681; 1651-1658 as patriarch) wanted to bring the Russian Orthodox Church and the Greek Orthodox Church closer together. For this reason, he changed a few rituals and texts in 1651. These changes were made to the Russian rituals to be closer to the Greek ones of Nikon's time. According to the Old Believers, Nikon did this without the support of the clergy. and without a council. When the changes were done, people who still practised the old rites were persecuted.