Aasimon are one of the four types of "Celestial", or angel, in the first three editions of Dungeons & Dragons. Characterized by their ability to be of Any Good Alignment and their appearance as humanoids with wings and fancy-colored skin, Aasimon are literally nothing more than Christianity-inspired angels, straight out of the general beliefs about them in the 80s, and just using a fancy name to disguise themselves.
Originally called Devas, that ultimately became the name of a specific subset of aasimon. The very first aasimon (the three Devas) appeared in Dragon Magazine #63's "Featured Creatures" article, with the Planetar and Solar appearing in the next issue. Both articles were written by Gary Gygax, and the Aasimon ultimately appeared in the Monster Manual II, released a year later in 1983.
Devas traditionally serve as the messengers of the Upper Planes. The most powerful of all, the Astral Devas, undertake missions to the Lower Planes. The less powerful Monadic Devas undertake missions to the Elemental Planes. Finally, the least powerful, the Movanic Devas, are charged with undertaking missions to the Prime Material.
Planetars and Solars are essentially the archangels of D&D, representing the most powerful servitors of all the Upper Planes. Their focus is predominantly on protecting the Upper Planes or missions of multiverse-shaking importance. The Solars are the very pinnacle of the Aasimon family tree, mightiest of all.
Light Aasimons are the weakest member of the family tree, and essentially equivalent to the Light Archons; a glowing mass of swirling vapor that seeks to support others in doing good by entering a familiar-like bond.
Agathions are described as the general warriors who protect the Upper Planes and serve under Planetars in the celestial armies. They're described as resembling very beautiful glowing elves.
In 5th edition, Aasimon were finally given the honest name of "angels" and were dropped back down to three types; a singular species of Deva, who serve as divine messengers; Planetars, who serve as instruments of divine wrath and vengeance, and Solars, who are direct servitors & stewards of the goodly gods.
|The inhabitants of the Planes of Planescape|
|Upper Planes:||Aasimon - Angels - Animal Lords - Archons - Asuras - Eladrin - Guardinals - Lillend|
|Middle Planes:||Formians - Githzerai - Inevitables - Marut - Modrons - Rilmani - Slaadi|
|Lower Planes:|| Alu-Fiends - Baatezu - Bladelings - Cambions - Demodands - Erinyes - Hags |
Hordlings - Imps - Kytons - Marilith - Obyrith - Succubi - Tanar'ri - Yugoloth
|Transitive Planes:||Astral Dreadnought - Githyanki|
|Inner Planes:||Azers - Elementals - Genies - Grues - Mephit - Salamanders - Sylphs|
|Sigil:||Dabus - Cranium Rats|
|High-ups:||Archangels - Archdevils - Archfey - Archomentals - Demon Princes|