Jango Fett and his advisors wanted to make a special unit of Clone Troopers that could handle special covert missions. The first attempt genetically failed however. This was resulted with the disobedient Null ARC troopers. The next batch was a success though, resulting in troopers such as Alpha-17, Clone Commander Fordo, ARC Commander Colt, and Alpha-66.
These troops were hand trained by Jango Fett personally and even learned a few of his moves. For example Captain Fordo adapted Jango's famous pistol spin. ARCs was dispatched to fight in the most dangerous missions imaginable during the Clone Wars. These missions were very crucial to the effect on the Clone Wars. For instance, Alpha (Alpha-17) along with Jedi Masters Obi-Wan Kenobi, Anakin Skywalker, and Shaak Ti saved the Clone DNA room from Separatist clutches in the First Battle of Kamino. After the Clone Wars and Jedi Purge the ARC Troopers were offered retirement from the Empire, some went into peaceful and quiet existence, some remained with the Empire, and trained Imperial Stormtroopers, a few became mercenaries and bounty hunters, and a few even joined the Rebel Alliance.
After Jango Fett was murdered the Kaminoans needed a new teacher for the ARC troopers. Alpha-17 came back and trained the new ARC troopers. Some of the troopers he trained were: Commander Cody, Captain Rex, Commander Fox, Commander Gree and most of the other clone commanders.