The difference between devils and demons in D&D is philosophy.
Demons, calling themselves tanar'ri, are chaotic evil and generally live in a lower plane, called "the abyss".
Devils, calling themselves baatezu, are lawful evil and generally live in a lower plane, called "baator", or Hell in simple terms.
Demons hate devils because they see devils as too lawful and tainted to be the true *evil* while devils see demons as tainted by chaos, and therefore not the real *evil*. They fight eternally, in something called The Blood War, to decide which side will then lay standing and be the one true source of evil in the multiverse.