Imperial Daleks are Daleks that are supposedly loyal to the Dalek Emperor. After the death of the Original Dalek Emperor at the start of the Dalek Civil War, a New Dalek Emperor came to power. This was actually Davros, the original creator of the Daleks, and he had been experimenting on a new form of Dalek which was deemed disgusting and twisted by the original Daleks, who believed themselves to be supreme.
The original grey Daleks rebelled against the 'Emperor' and his Imperial White Daleks. They became known as the Renegade Daleks, whilst the Imperial Daleks took control of Skaro and the majority of the Dalek fleet.
The Imperials waged war with the Renegades for many years, Skaro itself was abandoned by the Renegade Daleks but the Imperial Daleks did not want to set up their primary base there as they feared a Renegade Dalek trap. Instead, they set up their base on the desert planet of Necros instead. The Renegades attacked Necros in force but were driven off the planet by the superior fleet tactics of the Imperial Daleks. However, Renegade ground teams destroyed the Imperial Dalek base and killed all the Daleks there. To compensate, the Imperials landed their forces on Necros in an attempt to drive the Renegades out but the Renegades launched a new superweapon - a singularity drive - that destroyed the planet and wiped out the Imperial Dalek fleet there. The remaining Imperial Daleks were then forced to settle on Skaro.Near the end of the war, the Renegade Dalek leader, the Supreme Dalek, travelled to Earth in 1963 to recover an ancient Time Lord Weapon, the Hand of Omega. Davros sent a platoon of Imperial Daleks to fight the Renegades there, but later arrived in the Dalek Mothership after news that the Renegade stronghold on the planet of Antalin had been destroyed, and the war was practically won.
The Supreme Dalek thought little of this, however. The Imperials had no idea that he himself had fled the battle of Antalin to recover the Hand of Omega, since he knew it would win him the war instantly. He hadn't counted on the Imperials following him, however, and soon war broke out on Earth. Initially, the Renegades came off best, decimating the Imperial forces on the ground.Davros ordered the Special Weapons Dalek into position to annihilate the Renegade Daleks and any Humans that got in the way. The Renegade Daleks were defeated by this monster and the Hand went to the Imperials. The Supreme Dalek, however, survived.
Davros, in all his idiocy, attempted to activate the Hand of Omega, only for it to go haywire and break free of its housing. The Doctor contacted Davros and told him that he had reprogrammed the Hand, and that Davros was an arrogant fool for attempting to use it. Suddenly afraid, Davros attempted to destroy the Hand, but it fled the Dalek Mothership.
The Hand flew to Skaro where it obliterated the surface, utterly annihilating the Imperial Dalek fortress there and effectively equalising the forces of the Imperials and the Renegades. Without reinforcements, the Imperials charged the Renegade fleet, but the Hand came back for more. It destroyed both fleets of ships, negating both armies and ending the war. The Doctor later used this information to cause the Supreme Dalek to self-destruct, and Davros barely escaped with his life.