A story's exposition is the background details of the character(s), setting, or other elements of the story. In other words, we can say that a story's exposition tells us the details of the story before the current scene, like details about the past or anything that can give us information about the story.
A flashback is about past events, memories, while the setting only revolves around the scene, the time, or place of the story. A character is the "who" of the story.
Thus, the element of a story that tells us the "who, what, when, and where" is the exposition.
