MMORPG stands for Massively Multiplayer Online Role-Playing Game such as Runescape. A key difference between MMORPGs and regular RPGs includes the online aspect. RPGs don't need to have an online aspect and are meant as an individual adventure. MMORPGs revolve around having many players, usually working together. I would say RPGs are more story-driven than MMORPGs, not to say that there isn't a story in them.
PUBG doesn't really have role-playing aspects since it's mostly focused on combat. But I guess there is an MMO aspect to it, maybe not in the traditional sense, since there can be many players online.