After an assassination attempt on Senator Padme Amidala,a sabre dart used to kill the would be assassin Zam Wessel leads Obi-Wan Kenobi to the planet Kamino where he found an army of clones, supposedly ordered by a now deceased Jedi Master for the Republic.
Numerous brigades of clones have existed here are some of the most notable ones
327th Star Corps
Under the command of General Aayla Secura, Commander Bly's 327th Star Corps served the Republic valiantly, but when Emperor Palpatine gave Order 66 they turned their guns away from the Confederacy and towards Jedi Master Secura, gunning her down in cold blood.
Initially under the command of Anakin Skywalker and the clone Captain Rex, the 501st is quite possibly the most elite clone brigade in history, the have had many leaders aside Captain Rex, Commander Appo, Commander Vill, and Commander Bow to name a few. The 501st are better known as "Vader's fist" they are Darth Vader's personal strike force in the days following the Empire's rise.
The most elite Clone Troopers in the history of the Republic. The Red one is Sev. The Green one is Fixer. The Yellow one is Scorch. The Orange one is Boss the Team leader.