Terror Troopers were part of the Imperial Terror Soilders, an advanced and specialized unit within the Imperial military. Terror Troopers were given a unique cloaking device, specialized armor, a stun blaster, and unique combat training. They were very resistant to lightsaber attacks, Force - based attacks, and Mind tricks. Because of their training, they were often sent into hostile enviornments to dispose of their targets, such as enemy starships.
Terror Troopers sent into the 'Salvation' were ordered to escort Fett to the bridge (so Fett could capture Juno Eclipse), kill most the ship's patrols, and distract Starkiller long enough for Fett to reach the 'Salvation's' hangar and escape with Eclipse, then head back to Kamino.
After the Battle of Kamino, and the destruction of Timira City, the cloning of Terror Troopers was haulted.
Terror Troopers usually worked in small groups usually three to four. They sometimes work with Stormtroopers, but almost never seen with Imperial riot troopers.
After the Battle of Endor, Emperor Damascus used many Terror Troopers in his Galactic Empire.