Skaro was a planet of roughly the same mass and dimensions as Earth, and the twelfth planet from its sun. It had a single continent that was divided into halves, west and east, of almost equal size.
Skaro was the home of two sentient species, the Kaleds and the Thalss. Both species went to war with each other. The resulting mutations set in motion by the nuclear, chemical, and biological weapons used were soon seen by Kaled scientists as being the end of their species in its (ideologically defined) "pure" form. The project to preserve their race led to the invention of the successor species to the Kaleds, the dreaded cyborg Daleks.
The Thals were driven from Skaro by the Daleks. The inheritors of Skaro ruled a wasteland planet. Its seas were dead, and its surface was devoid of almost all life. Skaro was rich in minerals with which the Daleks began to build armies to conquer and destroy other intelligent beings on other worlds.
Skaro was destroyed sometime in the 27th century through the actions of the Seventh Doctor (portrayed on screen by Sylvester McCoy).
Although the Daleks are well known for their lack of jurisprudence (to whit "You are an enemy of the Daleks. You will be exterminated"), there have been two occasions on which they have taken enemies back to Skaro for a "trial" rather than killing them on the spot; the first was their creator Davros in Revelation of the Daleks, and the second was the renegade Time Lord known as the Master in the Fox telemovie. Neither trial occurred on-screen, so it is not clear what was actually involved; one thinks of the show trials of totalitarian regimes rather than the workings of the modern justice system. (The Master's trial presumably took place before the destruction of Skaro, although the Doctor only learned of it later.)
According to John Peel's The War of the Daleks, Skaro was not in fact destroyed. The Dalek Prime, leader of the Dalek race, altered history and reprogrammed Davros's memories to lead him into believing that the planet Antalin was Skaro. Skaro, apparently, still survives.
Doctor Who episodes that take place on Skaro or feature Skaro prominently: