Guardian of Time

The Guardians of Time, usually simply refered to as Guardians and collectively known as the Six-Fold God, were transcendental beings entrusted with aspects of the universe.

Guardians

 * White Guardian of Light and Order
 * Black Guardian of Darkness and Chaos
 * Crystal Guardian of Dream and Fantasy
 * Red Guardian of Justice and Truth
 * Azure Guardian of Equilibrium and Balance
 * Gold Guardian of Life and Death

Nature and power
Physically, the Guardians existed in Calabi-Yau Space. Together the Guardians were the Six-Fold-God of the Six-Fold-Realm. (PROSE: The Quantum Archangel) Though the Guardians seemed to be above technology, the Fourth Doctor mentioned "Guardian technology" in reference to the first Key to Time, (TV: The Armageddon Factor) which existed at every point in time, which K9 stated was a higher scientific concept. (PROSE: The Well-Mannered War)

The Guardians were also considered to be immortal. The White Guardian explained when Turlough believed the Black Guardian destroyed that they would always exist until the universe no longer needed them. The Black and White guardians also seemed to draw strength from their respective elements, becoming stronger or weaker depending on the state of their elements in the universe. The Black Guardian was most powerful in the far future, at the end of the universe where his counterpart could not interfere. (TV: Enlightenment/ AUDIO: The Destroyer of Delights/PROSE: The Well-Mannered War)

The Eighth Doctor claimed that the Guardians did not have the power to stop the universe from expanding. (PROSE: Dominion) However, the Seventh Doctor stated that they were capable of altering time on a grand scale and that the power they represented could not exist on something as small as a planet. (PROSE: Blood Heat/COMIC: Time & Time Again) Additionally, they could pull the arms off spiral galaxies and were one of the powers of the Omniverse which could do anything. He implied that the Time Lords might have made a deal with them because of this power. (PROSE: No Future)

The Guardians possessed the power of transmogrification, being able to change their shape at will, (TV: The Armageddon Factor, Mawdryn Undead) and could supposedly alter time so that "nothing would ever happen" to a person. Though the White Guardian was able to summon the Doctor's TARDIS, cut it's power, and induce time override, (TV: The Ribos Operation, Enlightenment) the Black Guardian was affected when its defensive systems were fully activated, reshaping into his negative form for unexplained reasons. He could also be dispersed by releases of energy such as the breaking of the Key to Time, (TV: The Armageddon Factor) the shorting-out of time differential, (TV: Mawdryn Undead) or contact with Enlightenment. He could also channel his power through an individual, follow the Doctor along his past and future, and both he and his counterpart were able to enter the TARDIS. They were also able to move individuals, such as Romana I and Stokes, through time and space, and send the Eternals back to Eternity. (TV: The Ribos Operation, Enlightenment/PROSE: The Well-Mannered War)

Interactions with other beings
The Guardians originated in another universe. Rassilon himself only knew of two Guardians, the twin opposed pairs representing Black and White, and also considered the Great Old Ones "sub-Guardians". (PROSE: Divided Loyalties)

Because of their immense power the Guardians were not bound by physical laws but rather codes of conduct as were the Chronovores and possibly other transcendental beings. (PROSE: No Future) This would explain why the Black Guardian "must not be seen to interfere." (TV: Mawdryn Undead)

The Eternals also knew of the Guardians of Time and respected them greatly, calling them "Enlighteners." (PROSE: The Quantum Archangel) They set up the games that kept the Eternals amused. (TV: Enlightenment)

The Guardians used other beings as willing or unwilling instruments. The White Guardian sent the Fourth Doctor and his companions on quests for the two Keys to Time (TV: The Ribos Operation, COMIC: Time & Time Again), while the Black Guardian assigned the Shadow to stop him (TV: The Armageddon Factor). The Guardians were themselves agents used by the Grace. (AUDIO: The Judgement of Isskar, The Chaos Pool)

The Black Guardian employed another agent, Turlough, (TV Mawdryn Undead) to kill the Fifth Doctor and later, with the White Guardian, sponsored a contest for a group of Eternals to win Enlightenment. (TV: Enlightenment) The Crystal Guardian, however, simply pursued private games. (PROSE: 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. (PROSE: The Quantum Archangel)

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

A fictional Beige Guardian once appeared to force the first eight incarnations of the Doctor to defeat all their past enemies at once, but it was actually just a video game Izzy Sinclair was playing on the Time-Space Visualiser aboard the Doctor's TARDIS. (COMIC: Happy Deathday)

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, and still later continuity made him an Elder God. Hecuba (called the Queen of Time) was introduced as his sister, so whether she was a Guardian or how she was connected to them or their role was not specified.
 * Until Divided Loyalties and The Quantum Archangel, reference to the Guardians only consisted of Black and White, and to date, they are the only Guardians to be seen, depending on what the Celestial Toymaker really is.
 * Because of their similarities to other transcendental beings, it could be that "Guardian" was a position held by entities rather than a species.