Dale Sullivan
Genres are types of discourse. Modern theory defines rhetorical genre as a way of acting in situations. Carolyn Miller, for instance, defines rhetorical genres as "typified responses" that come out of "recurrent situtations." This definition is based on the same criteria Aristotle used when he divided rhetorical genres into three types (Read Artistotle's Rhetoric, Book I for more detail). This is a summary.
Forensic Rhetoric. Forensic rhetoric, according to Aristotle, is rhetoric in which people try to determine whether or not a past action was just or unjust. It's the sort of rhetoric we get in court cases or in investigative hearings. Arguments in forensic rhetoric are usually based on reasoning that goes something like this. We know that X happened; therefore, it is most probable that Y also happened. Aristotle said, for instance, that if a person has motive and opportunity, he will do the deed; therefore, if you can show that person had motive and opportunity, you can deduce that he did the deed.
Deliberative Rhetoric. Deliberative rhetoric is rhetoric used to determine what the most expedient course of action is for the future. This is the kind of rhetoric that occurs in legislatures or in planning committees. Dileberative rhetoric can be summed up this way--it asks, "What should we do in this situation? or What should we do to achieve this end? or What should we do to solve this problem?" Notice that it always asks, "What should we do?" Arguments in delberative rhetoric are based primarily on examples from the past. If the current situation is like a past situation, you can argue that we should do X because in situation Y they did X and it was effective. Much modern deliberative rhetoric appeals to statistical analyses of probability.
Epideictic Rhetoric. Epideictic rhetoric, sometimes called demonstrative or ceremonial rhetoric, focuses on the present and praises or blames people, cities, policies, etc. Its primary purpose is to celebrate the values held in common by a community or culture. Although it draws on the past and influences the future, it is not trying to solve a current problem (like was justice done in this case, or what should we do now). Instead, it magnifies the virtues of people or cultures by pointing to specific instances from the past.