The way I do it is by establishing the factions or individuals involved, including their goals and motivations, and the figuring out what might happen if the PCs don't get involved. This makes it much easier to decide what the "bad guys" do in response to PC actions, and has the benefit of having information in place for when they don't. Because sometimes they don't.I get this in concept, but are hung up thinking if I do this or what the difference is with situations and encounters and plots. I might have a tribe or orcs planning to attack the village. I plan out an encounter where they attack and their size and tactics. I plan on the PCs fighting, but guess it depends on them. I also plan on the orcs taking some prisoners and going back to a cave. I plan on the PCs going to save the villagers.
The plot of the adventure is to survive the attack and rescue the villagers before they are sacrificed. I have a few situations that the PCs walk into but they could also just not want to do any of it.
I guess I'm thinking that all of these words might just be the same.