Celestials are the beings whose collaborative effort created the Universe. There are six Celestials who are widely recognised as 'Major Celestials' and many other Celestials that also contribute to the Universe but are worshipped less widely.
Of the six major celestials only three are still living and active in the universe, those being Istari, Anastar and Seh'Makou, with Legion being imprisioned. These Celestials are worshipped on many of the older planets in the galaxy (such as Feyworld) and temples are erected in their honour.
Gaia - The Life-giver
Nozdormu - The Starforger
Zalgo - The Maddening (Formally The Light-Bringer)
Istari - The Wise
Legion - The Chaotic
Seh'Makou - The Cunning
Other Notable Celestials
These are either Celestials that are less worshipped throughout the galaxy or the "children" of the other celestials, created due to intense amounts of energy in the galaxy. In the case of the three Almis brothers they were once celestials but after being cast down in 1185AD they became Descendants.
Elijah Almis (Formally Nolstaris) - The Peace-Keeper
Ven Almis (Formally Zanatosh) - The Just
Anastar - The War-bringer, son of Legion
Curumo - The Skilled
R'tas - The Conservator