Me when I first got into D&D in 3.5: "Why are demons and devils different things? What's this about a Blood War? And what the hell are Yugoloths???"
Me sometime during 4E: "Oh, demons are mostly evil super monsters generated by the Abyss while devils want your soul, mostly to use it to make more devils to fight against demons. That's why Asmodeus is tolerated by the other gods; his devils are the main ones fighting the forces of the Abyss in the Blood War so that the forces of Good don't have to spend all their time doing it and risk being corrupted like the devils were. Plus, players can more easily make deals and make temporary alliances with devils, who will use the opportunity to try and make you see things their way while also trying to gain rank in their surprisingly fair meritocracy. Okay, neat, this all makes sense."
Me in 5E: "Yugoloths are back for some reason."
I think in my first campaign I just lumped demons and devils together, whereas in my first 5E campaign I made the division between demons and devils and the Blood War extremely important at the end.