Celestials, as a race, are good aligned outsiders; they are not native to the Material Plane. Celestials thoughts and actions are just and pure and righteous.
Celestials primarily speak Celestial among other tongues.
Whenever a Celestial and human successfully mate, the result is a half-celestial.
Later, after the celestial blood has been diluted a bit, the result is an aasimar.
Celestials, or at least archons, eladrins, and guardinals, are ruled over by the Celestial Paragons.
Celestials can occasionally be found on the Material Plane due to visiting it or working on behalf of their deity. However, their home plane is the Seven Mounting Heavens of Celestia.