It's been a while, but . . . .
Planescape often presented classic D&D baddies (and goodies) in atypical situations. Fiends weren't always evil, celestials weren't always good. That was interesting.
I think the idea of the Blood War started with Planescape, although perhaps it was pulled from earlier D&D lore. Mostly Planescape fleshed out the planes, including the lower planes, and gave us more demons and devils than we had before.
So, not terribly different from the classics, but more stuff, presented with a different tone.
By the time we got to Planescape, TSR was less afraid of angry mom's and of using terms like "demon" and "devil", but "tanar'ri" and "baatezu" were a part of 2nd Edition lore . . . so we started getting lawful and chaotic fiends that were outside of those categories. It seemed to be moving towards "tanar'ri" as the prominent family or race of demons, but there were others, like the obyrith. Same with the devils. Kytons were originally presented as fiendish, living in Hell, but not actually baatezu.
I miss those terms myself. I don't mind using "demon" and "devil", but . . . "tanar'ri" and "baatezu" are so much more evocative to me. My head canon sees those terms as what the fiends call themselves, and its mortals who use imprecise terms like demon or devil.