Though they may seem like they couldn't be more physically different, angels and demons are two creatures of the same essence, and collectively referred to as Celestials.
The difference comes from the intervening millennia since The Fall, and the energies of The Abyss which suffused those rebel celestials who resided there in those millennia.
The Angelic Hosts Edit
The Angelic Host were the soldiers of God. Since the rebellion of The Christ Child and the disappearance of God, the Host has turned to become the new master of Humanity. The masterless messengers are left to their own devices, and while some are well adjusted enough to either want to leave and be left alone, most see themselves as Humanity's rightful new masters, and treat the Meek as so much chattel, to fight in their armies, farm and cook and equip those soldiers, and make new chattel for them. They are stern and cruel in their austere demand for purity.
The Angelic Hosts take many forms, but are generally pristine, and beautiful, albeit in an alien and terrifying way at times. When they evoke animal forms and traits, angels use bird and mammalian forms almost exclusively.
The languages of the Angelic Host are Enochian and The Words of Creation.
The Hordes of Hell Edit
The Hordes of Hell were those angels who rebelled at the dawn of time for reasons that are unknown to man. The Angels have their story, and some demons have stories that line up, but not all. Some say they fell for their love of man and God's contradictory instructions. Some say they fell because Lucifer could run things better. Some say they fell simply because they got tired of taking orders. But no one is telling humankind which it is. Since the rebellion of The Christ Child and the disappearance of God, the hordes have run roughshod over the Earth. Some have gathered to them cults of followers, while others have fashioned themselves as warlords of the wastes.
The Hordes of Hell take just as varied forms as the Angelic Host, though whereas every Angel is beautiful, even those who are terrifying, every demon is terrifying, albeit sometimes in a beautiful, alluring way. Many demons evoke animal forms or traits, generally exclusively reptilian, arthropodic, or oceanic.
The languages of the Hordes of Hell are Enochian and the Dark Speech.