Gallifrey is the home world of the Time Lords in the Doctor Who continuum. Following the Last Great Time War, Gallifrey was believed destroyed, but it has been relocated to a pocket universe due to the combined efforts of all thirteen incarnations of the Doctor. A lot of people are pretty upset about this, including the various Time Lord agents. As of Series 9's two parter finale, Gallifrey has been found and is once again accessible by the PPC.
According to one of the last surviving Time Lords, the Doctor, Gallifrey had a burnt-orange sky, lit by two suns. The local flora was red. The primary city was the Citadel of the Time Lords, beneath the mountains of Solitude and Serenity, silver and gleaming inside its bubble (presumably a force-field). It was occupied by a lot of people in funny robes. The rest of Gallifrey was apparently a BBC-standard disused quarry (just like the rest of the universe), except for the valley with the tower and random monsters, but no one remembers The Five Doctors any more.