How demons and angels and such are "IRL" highly depends on which religion/culture you draw your inspiration from, but to answer your indirect question, the devils of DnD are kinda fallen angels, demons are another breed all together, and the fight between angels and the demons is what lead to the devils falling.
That sounds like the 4e interpretation of demons. In AD&D and 3e, devils aren't fallen angels as a general rule though some devils and Archdevils are indeed fallen celestials, and demons fight more with devils (with daemons aiding both sides and demodands mostly staying out of the Blood War) because the celestials (composed of angels, archons, guardinals, and eladrin) leave them to their own devices in the hopes that the fiends are kept occupied and ignore the Upper Planes and the Prime Material.
Better to DM in Baator than play in Celestia
You can just call me Dice; that's how I roll.
Spoiler: Sig of Holding
Originally Posted by abadguy
Darn you PoDL for making me care about a bunch of NPC Commoners!