Aristotle introduces the concepts of peripeteia (reversal of fortune) and anagnorisis (discovery or recognition) in his discussion of simple and complex plots. All plots lead from beginning to end in a probable or necessary sequence of events, but a simple plot does so without peripeteia or anagnorisis while a complex plot may have one or both of these elements. The peripeteia or anagnorisis of a complex plot should themselves be necessary or probable consequences of what came before so that they are a part of the plot and not unnecessary add-ons.
Peripeteia is the reversal from one state of affairs to its opposite. Some element in the plot effects a reversal, so that the hero who thought he was in good shape suddenly finds that all is lost, or vice versa.
Anagnorisis is a change from ignorance to knowledge. This discovery will bring love and happiness to characters who learn of good fortune, and hatred and misery to those who discover unhappy truths. The best kind of anagnorisis accompanies peripeteia. That is, a reversal of fortune effects a discovery or vice versa. For instance, Oedipus' discovery of who his mother is effects a reversal of fortune from proud king to horrible disgrace. Aristotle suggests that anagnorisis is possible by a number of other means as well, but it is most intimately connected to the plot when it accompanies peripeteia. The two together will help to arouse pity and fear and will also help to draw the play to its conclusion.
In addition to peripeteia and anagnorisis, Aristotle defines a third part of the plot—suffering—as actions of destructive or painful nature, such as murders, torture, and woundings.
In Chapter 12, Aristotle discusses the quantitative elements of tragedy—the different parts of the performance. These are the Prologue, Episode, Exode, and a choral portion consisting of Parode and Stasimon. In addition, some tragedies have songs from the stage and a Commos, a lamentation sung by both actor and chorus. The Parode is the first full statement of the chorus; everything that precedes it is Prologue. The Stasimon is a choral song in a certain meter, while action that takes place between choral songs is Episode. Everything that follows the last choral song is Exode.
Peripeteia and anagnorisis are fancy Greek words, but we are all quite familiar with the concepts. Anyone who has watched the eighties television show The A-Team is quite familiar with peripeteia. Every episode, the A-Team thinks they have the bad guys stumped, but then the tables are turned (the first peripeteia), and the team is captured. Of course, the bad guys always lock them up in a warehouse full of welding equipment, and the A-Team builds a big machine, breaks out, and busts the bad guys (the second peripeteia). This example may seem silly, but the point is that peripeteia is not an archaic concept but an incredibly potent literary device that is used effectively in almost every genre at almost every level.