Also Ukrainian kozaks greatly influenced the formation of Russian cossacks (KAZAK in Russian).
Those had their share of fame much later on in 19-20th centuries.
Around half of Russian cossacks are Ukrainian by blood.
Historically, the Cossacks were predominantly Russian Orthodox Christian, but there were a few, especially around Crimea, who were Muslim, and some were even Buddhists from Mongolia.
Most early accounts of the Cossacks come from those who fought them, and recount their brutality and the use of bull whips as a battlefield weapon. Later accounts come from Russian officers who fought alongside the Cossacks during Napoleon's invasion of Russia. The Cossacks fought alongside the Russian army as late as WWI. In the Revolution, the Cossacks aligned with the Czar, and committed numerous acts of brutality against anyone suspected of Communist sympathies. They also carried out pogroms against Jewish villagers, under the assumption that all Jews were Communists. After the Revolution, the Cossacks were persecuted by the Bolsheviks and finally crushed by Stalin.
The Cossack cavalry chased the Austrians down the embankment with their whips.
The Cossack cavalry chased the French into the marshes with their sabers.
A Cossack can kill three ninjas and seven pirates with one lash of his whip.