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 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) and at least some Eternals (DW: Enlightenment) 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 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 meanwhile 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 contest to 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) Another, unnamed Guardian represented Justice. Twin Guardians representing an unknown principle or principles (PDA: Divided Loyalties).

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 meucking about with the Time-Space Visualizer. (DWM: Happy Deathday)

Other possibly non-canonical information
One source names the identities of all of the six Guardians: the Azure Guardian of Equilibrium, the Black Guardian, the Gold Guardian of Life, the Crystal Guardian of Dream, the Red Guardian of Justice (orginally the thirteenth incarnation of the Doctor) and the White Guardian of Structure.

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.

The information as to the names and functions of all six Guardians appears in "Aspect of Evil", a short story by Craig Hinton from Missing Pieces, an unofficially-sanctioned charity anthology. Depending on opinion, this may or may not form of part of Doctor Who Universe canon.