Guardian of Time

The Guardians of Time, collectively known as the Six-Fold God were Transcendental Beings entrusted with aspects of the universe.

Guardians

 * Black Guardian of Darkness and Chaos.
 * White Guardian of Light and Order.
 * Crystal Guardian of Dream.
 * Red Guardian of Justice.
 * Azure Guardian of Equilibrium.
 * Gold Guardian of Life.

Nature
Physically, the Guardians existed in Calabi-Yau Space. Together the Guardians were the Six-Fold-God of the Six-Fold-Realm. (PDA: The Quantum Archangel) Though usually the Guardians would seem to be above technology, the Doctor made reference to "Guardian technology" in reference to the first Key to Time. (DW: The Armageddon Factor) The Guardians are also immortal or as close to immortal as a sentient being can get; the White Guardian explained after Turlogh believed the Black Guardian was destroyed was that they would always exist until the universe no longer needed them. The Black and White guardians also seem to draw strength from their respected elements, becoming stronger or weaker depending on the state of their element in the universe.

Interactions with Other Beings
The existence of the Guardians was known to the Time Lords. (DW: The Ribos Operation) Rassilon himself only knew of two Guardians, the twin opposed pairs representing Black and White. Rassilon also considered the Great Old Ones "sub-Guardians". (PDA: Divided Loyalties)

The Eternals also knew of the Guardians of Time and respected them greatly. (PDA: The Quantum Archangel) They set up the games that kept the Eternals amused. (DW: Enlightenment)

The Guardians have used other beings as willing or unwilling instruments. The White Guardian set the Doctor and his companions on a quests for the two Keys to Time (DW: The Ribos Operation, DWM: Time & Time Again), while the Black Guardian assigned the Shadow to stop him (DW: The Armageddon Factor).

The Black Guardian would employ another agent, Turlough (DW Mawdryn Undead) to kill the Doctor and later, with the White Guardian, sponsored a contest for a group of Eternals to win Enlightenment. (DW: Enlightenment) The Crystal Guardian, however, simply pursued private games. (PDA: Divided Loyalties) They could also act together to directly re-shape creation as the Six-Fold God, as when they retroactively un-did the very existence of Prometheus. (PDA: The Quantum Archangel)

Known Guardians
The Guardians numbered six in all (PDA: Divided Loyalties). The Black Guardian represented Darkness and Chaos. (DW: The Ribos Operation) The Crystal Guardian represented Dream. (PDA: Divided Loyalties) The White Guardian represented Light and order. (DW: The Ribos Operation) The Red Guardian represented Justice, the Azure Guardian represented Equilibrium and the Gold Guardian represented Life. (PDA: The Quantum Archangel)

The Beige Guardian supposedly made the Doctor's life miserable by putting him on a stupid quest to fight his past enemies in all his incarnations so far. Probably, though, this only occurred as a result of Izzy Sinclair mucking about with the Time-Space Visualiser. (DWM: Happy Deathday)

One source states that the Red Guardian of Justice was orginally the thirteenth incarnation of the Doctor. (CP: Aspects of Evil (Missing Pieces))


 * Depending on opinion, the Missing Pieces anthology may or may not form of part of Doctor Who Universe canon.

Behind the Scenes
The existence of the Celestial Toymaker as a character on Doctor Who pre-dates the introduction of the Guardian mythos. Later continuity retroactively made him a Guardian.

As of yet, the Guardians have not made an appearance in the new series, though it was believed by some that the woman in the End of Time was a form of the White Guardian.

Rumors of a silent dark enemy stalking the Doctor and Amy makes it seem possible the Black Guardian may appear this season.