The current Gallifreyan civilization was founded at an unknown time in the distant past by Rassilon, whose legacy is seen throughout the entire culture and technology of the Time Lords. Perhaps the most significant of these legacies is the Eye of Harmony, an artificial black hole singularity created by his colleague Omega and captured by Rassilon for use as a power source for time travel. All Time Lord TARDISes draw their power from the Eye of Harmony.
Gallifrey is surrounded by an impenetrable force field called the "transduction barrier." This prevents all outsiders (with hostile intent or otherwise) from approaching the planet. It also allows the Time Lords to maintain their status of absolute neutrality, allowing them to observe the actions of the rest of the Universe without actually taking part in its affairs. The barrier was breached once during the course of the show, by the Sontarans, using an inside agent.
The planet's civilization is among the most powerful in the universe yet is strictly neutral and only observes. For certain time Lords this proves impossible and some have left over the years to actively help or hinder the Universe. Renegades like The Rani, The Meddling Monk and The Master interfere for power or just because they can. The legendary Doctor does so to produce the greatest good he can.
Warning: Spoilers follow
In "The Ancestor Cell", the planet is destroyed as a result of the Doctor's desire to prevent the voodoo cult Faction Paradox starting a war against the unnamed Enemy.