Guardian of Time

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

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)

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 Time. (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. (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.