Peter the Great and Ivan the Terrible were both Russian czars. Peter the Great was the first CZAR of the Russian Empire, reigning from 1682-1725. He is credited with modernizing Russia and expanding its borders. Ivan the Terrible was the first tsar of all Russia, reigning from 1547-1584. He is known for his violent reign and for unifying the Russian people. The term 'CZAR' is a four letter word used to refer to either of these two rulers.