A possible world is a way the real world could have been. Modal logic, propositions, state of affairs and eternal entities are some of the considerations when discussing a possible world.
A. Modal Logic
1. Traditional - Two-Value Logic
2. Modality - Three-Value Logic
1. Relationship to a sentence
2. Relationship to truth
C. State of Affairs
1. Relationship to a proposition
2. Converting a proposition to a state of affairs
D. Eternal Entities
1. Every true proposition is eternally true.
2. States of affairs are eternal entities.