"Plot" is a narrative device that generally means the events that make up a story. There are numerous different ways in which a writer will develop plot, and the most common form is through sequence, although sequence can be shifted in order to highlight suspense or dramatic tension in a story.
In the Poetics, Aristotle said that plot was the most important element of any story. All plots, according to Aristotle, need a beginning, middle, and end. In the 19th century, Gustav Freitag, a novelist and playwright, developed the pyramid of narrative structure. This scheme divides a plot into five parts: exposition, rising action, climax, falling action and resolution.
Exposition introduces the characters and the setting. Next, conflict is introduced and this begins the phase of rising action. Events continue to build until they reach the climax, the dramatic turning point of a story. As the characters resolve the conflict, this introduces the segment of falling action, which traditionally leads to a happy ending. Plot ends at the denouement, the resolution of plot.
A plot device is any means of advancing the story, as contrasted with narrative technique. Familiar plot devices include the MacGuffin (an object of pursued desire, such as the Heart of the Ocean necklace in Titanic), the red herring (a misdirection) or Deus ex machine, when a new event or character suddenly solves the conflict.