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.