The 501st Legion was the mainframe unit of the Imperial Army. Almost all troopers served under Darth Vader in the 501st Legion.
The 501st was a Clone Trooper unit that served under Jedi Knight Anakin Skywalker during the Clone Wars. This unit was originally made up of clones either dressed in the standard white armor or with blue markings. Later, after Anakin turned to Vader, the 501st became a mostly Clone Stormtrooper unit with occasional human recruits. Later with the death of Vader and Sidious, the 501st had lost a majority of its clone units and began accepting more human recruits into the army.