Originally, there were two races living on Skaro. The Kaleds, who all had black hair, and the Thals, who all had blonde hair. One day, Davros, a young Kaled who was a bit odd, said: those blondies over there should die!
And war was waged for a thousand million years. War that reduced the once technologically advanced races to mere scavengers, using trench warfare, rifles and gas masks to fight with. Davros kept himself alive all this time using a life supprt machine, and he noticed some odd changes from when he was a boy. For a start, Skaro was now radioactive, and anyone who went outside without adequete protection was hideously mutated. Davros accelerated these mutations to create a twisted creature, he called it a Dalek.
Placing the Daleks in travel casings, Davros called his creations the perfect form of life. He removed all their emotions, except hatred, and fitted them with deadly weapons. He sent them against the Thals who were utterly annihilated, but in a final act of desperation, the Thals launched a missile at the Kaled city, wiping it out completely. Only Davros and his Daleks survived.
They abandoned Skaro until many years later, around the time of the Dalek Civil War, the Renegade Daleks set up a base here, but were driven off the planet by the Imperials. The Imperials did not set up a base on Skaro, instead opting to use Necros as a base instead. However, when Necros was destroyed by Renegade Ships, the Imperials were relegated to Skaro.
Skaro was razed by the Hand of Omega after the Doctor reprogrammed it to annihilate all traces of the opposing factions of Daleks. The surface of Skaro was burnt to a cinder, but this actually benefitted the planet as it finally blew away the radiation that had been festering there for millions of years. Skaro was terraformed by the Hand, and centuries later trees and plants were transplated there by the League.