A battlefield on avernus, demons on the left fighting devils on the right
Devils
Loosely based on myths
In many cultures devils are just another way of saying demons, but some religions have one devil that rules all demons, but in general "devil" is a personification of evil. The word devil comes from the middle English language were it means slanderer, or to slander. In most depictions, there is one devil that rules over demons in hell, this devil has many different names throughout cultures. Some examples include: Lucifer, the angel of betrayal, Satan, Beelzebub, Mephistopheles, and so forth. Devils in mythology are usually depicted as humanoid creatures with either bat wings, curled horns, blue or red skin, or having some sort of pig or goat body parts, they are also depicted as having large burning pitchforks. In Dungeons and Dragons, the names of the devils that I listed all have a place in the realm of the nine hells as Arch-devils.
Imps are the weakest, and smallest among devils
horned devils are the mid level ranks in the infernal hierarchy
pit fiends are the most powerfull non archdevils
Asmodeus, the god of Devils
From mythology
Medieval christian devil painting by Jacob D. Backer
Depiction of Asmodeus from the Dictionare infernal
Beezlebub from the demonology