The Celestis were the Celestial Intervention Agency. During the Second War in Heaven they removed themselves from the universe to escape the temporal damage.
Nature
The Celestis removed themselves from history and detached themselves from the physical plane of existence, making themselves ideas, and so harder to destroy (harder than a physical body tied down to the timeline).
They inhabited a world outside the normal space time called Mictlan, a place constructed using the Matrix. It existed outside the physical universe.
The Celestis were split on whom to support during the War: the Time Lords or the Enemy. (PROSE: Alien Bodies)
Behind the Scenes
Rassilon's plan for the Time Lords to survive the Ultimate Sanction at the end of the Last Great Time War by becoming "creatures of pure consciousness" resembles the actions taken by the CIA during the Second War in Heaven in becoming the Celestis. (TV: The End of Time)