Ah, Star Wars. Throughout the history of science fiction, very few movie series can come close to the mystery, intrigue, and fantastic planet locales of this extraordinary universe. George Lucas probably had no idea in the 1970's when he first started writing the screenplay for 'Star Wars Episode Four: A New Hope' that he would intentionally secure himself a place among the legends of science fiction and fantasy in media for eternity.
Clones are humans that have been bioengineered for battle who can be grown from human cells rapidly - in this case from the legendary infamous Mandalorian bounty hunter, Jango Fett, father of the infamous bounty hunter and clone, Boba Fett. Sometimes they even developed distinct personalities, though those who did during training on Kamino were later eliminated. All but five of these clones were not, and later most of them defected from the Old Republic before and after Order 66 was given by Senator (Emperor) Palpatine aka Darth Sidious.
Prior to this infamous order, clones in question were assigned by Palpatine to be military advisers to high ranking Jedi on Coruscant and in the universe during the Clone Wars. But the non Kaminoan clones would become the templates for the very first stormtroopers of the Galactic Imperial Empire. They were first seen in the weeks and months prior to Order 66. The few Jedi, besides Obi Wan Kenobi and Yoda, who survived the Great Jedi Purge were actually assisted off-planet by sympathetic senators and renegade clones who broke free of their programming and defected from the Grand Army of the Republic.
However, most of these clones who followed this order (supposedly after the stormtroopers were created) were eliminated by Emperor Palpatine because he was convinced that humans were, are and forever would be the superior species in the universe and as such, clones were inferior and only bred to follow his commands.
Those who did not were deemed renegades who would mostly end up on Mandalore and not only discover a way to reverse their rapidly genetically engineered aging, but even have families and help Jedi refugees in hiding from Imperial deathmarks.