Most students of literature know what a genre is–it’s the name given to a work based on its formal, technical, or sometimes content characteristics. Common genres are epic, comedy, drama, tragedy, novel, short story, poem, etc.–in other words, definitions of literary categories based on how the work is built, the form it takes, the similarities it has to other works. But Chaucer wrote in a period when the concept of genre was much different than what it is now. While today we use genre as a structure word or description of form (something is a poem or a play or a novel, etc.); to the late Middle Ages, genre was a content description. Here is a rough breakdown of the most obvious genres in which Chaucer wrote:
Romances explore the notions of love and war, usually in a refined setting. They revolve around highly idealized behavior and courtship. Women basically act beautiful and remote; men try to impress the women with their prowess and virtue; the course of true love never runs smooth.
The Knight's Tale, the Squire's Tale, Sir Thopas, the Wife of Bath's Tale, the Merchant's Tale, the Franklin's Tale, the Book of the Duchess, Anelida and Arcite, the Parliament of Birds, Troilus and Criseyde, and the Prologue to the Legend of Good Women are wholly or in part romances. In addition, most of Chaucer's short poems explore notions from the romance culture.
Tragedies were usually prose or poetic narratives, not dramas. In the Middle Ages, tragedy was perceived as a reversal of fortune, a fall from a high position. This view of tragedy derives from the Medieval concept of fortune, which was personified as Dame Fortune, a blindfolded woman who turned a wheel at whim; people were stationed at various places on the wheel--the top of the wheel represented the best fortune, being under the wheel the worst fortune. However, the wheel could turn suddenly and the person on top could suddenly be under the wheel, without warning. Thus there is not necessarily a "tragic flaw" in a medieval tragedy; all people's lives are subject to the turns of Fortune.
Fabliaux deal with an extended joke or trick (frequently bawdy), and, as befitting such "low" behavior, take place among the lower classes of society. The authors were probably clerics, lerned writers making fun of what they perceived to be the customs of the lewed. In many fabliaux, clerks take advantage of less-learned people for comic effect. Many critics see fabliaux, with their emphasis on crude sex and physical comedy, as a mirror-image of the romances; they turn each other inside out.
The Miller's Tale, the Reeve's Tale, the Cook's fragmentary Tale, The Friar's Tale, the Summoner's Tale, the Shipman's Tale, and The Canon's Yeoman's Tale are all in the fabliaux tradition. The Wife of Bath's Prologue, the Merchant's Tale, and the Nun's Priest's Tale all use fabliaux elements.
Dream Visions are stories where the narrator or protagonist receives guidance through a dream, thus bypassing medieval restrictions on fiction. The source of this dream might be God (a truly prognostic "visio"), the devil (sometimes a form of sexual temptation like an incubus or succubus), or natural causes. Typically, the dream vision occurs in a predictable series of stages: 1) the dreamer falls asleep in the midst of some life crisis or emotional impasse; 2) the dreamer, almost always a male, finds himself in a beautiful natural place (locus amoenus), often an enclosed garden filled with beautiful plants, animals, etc. (hortus conclusus); 3) the dreamer encounters a guide figure who instructs the dreamer and/or leads the dreamer to one or more allegorical visions; 4) the dreamer may interrogate the guide figure about the significance of the visions, but often this does not produce satisfactory results; 5) something within the dream causes the dreamer to awaken before the full significance of the dream can be explained.
Chaucer's three surviving dream visions (Parliament of Birds, Book of the Duchess, and House of Fame) are characterized by some additional stylistic quirks: the dream involves a book by some classical author; the nature of the dreamer's problem almost always remains a mystery, and the problems of the vision's characters occupies the center of our interest; the vision usually appears to contain both a moral/social allegory and references to contemporary political events (courtship of Richard II's bride; death of John of Gaunt's first wife; struggles between parliamentarian and royalist forces). Both Pearl and Piers Plowman are significant dream visions that may well have been known by Chaucer's audience.
Sermons are exhortations to abandon vice and turn to virtuous living. These were the most common kind of public discourse in Chaucer's time, and a genre even the illiterate would understand and know how to interpret. Like today's televangelists, medieval preachers were skilled rhetoricians who knew how to reach their audiences. Typically a sermon included a) a statement of the "text," usually a Bible passage; b) relating that text to a theme (e.g. gluttony, Christmas, etc.); and c) a series of stories and proverbs, some classical, some contemporary, some in Latin, some in the vernacular, to illustrate the theme.
The Pardoner's Prologue and Tale, the Nun's Priest's Prologue and Tale, and the Parson's Tale are all based on sermon techniques. These techniques also figure in the Friar's Tale, the Summoner's Tale, the Clerk's Tale, the Man of Law's Tale, the Wife of Bath's Prologue and Tale, the Physician's Tale, the Manciple's Tale, and Melibee.
Saints' Lives are often religious versions of romances, with holy men and women figuring as the main characters. The chief difference is that the motivating force is not love for a human but love for God. These feature weaker characters overcoming stronger ones (often women defeating men) through the agency of their faith. Prayers and "complaints" are addressed to Christian deities, not pagan ones.
Saints' lives are the dominant genre in the Clerk's Tale, the Man of Law's Tale, the Physician's Tale, the Prioress' Tale, and the Second Nun's Tale. Elements of the saints' life also figure in the Knight's Tale, the Franklin's Tale, The Wife of Bath's Prologue and Tale, Troilus and Criseyde, and the Legends of Good Women.
Penitential Tracts are those in which a penitent looks over his or her past life, recalling its events, and promising to repent for his or her sins. Sometimes a confessor guides the penitent in this self-examination. Occasionally the story takes the form of a confession of a vice-type character, who reveals his or her techniques and flaws in the telling of the story.
The Parson's Tale is a full-scale penitential tract; confessional elements also appear in the Wife of Bath's Prologue and Tale, the Pardoner's Prologue and Tale, and the Canon's Yeoman's Prologue and Tale. It can be argued that in a sense, all the Canterbury Tales are confessional.
Moral and Didactic Writings are those whose purpose is chiefly instructional. Sometimes they are called exempla (singular exemplum), because they were used as examples from which people could learn virtuous conduct. They were very popular in Chaucer's time (some think because of the vivid examples of "bad" behavior), even if they are sometimes less appealing now. Good triumphs over evil in the end, but evil, as Chaucer's contemporary Charles d'Orleans observed, has all the fun.
The Monk's Tale, Melibee, The Man of Law’s Tale and the Parson's Tale are based on moral/didactic structures. Substantial elements figure into the Knight's Tale, the Merchant's Tale, the Nun's Priest's Tale, the Parliament of Birds, and Troilus and Criseyde. Chaucer also translated three of the "standard" moral guides of his time, Boethius' The Consolation of Philosophy [Boece], Pope Innocent’s On the Wretched Condition of Mankind, and the French Romance of the Rose. The Treatise on the Astrolabe and the Equatorie of the Planets are examples of secular didactic writings in this tradition.
As you can see, Chaucer blurs the generic distinctions in his works by using elements from two, three, or even four genres in most works. This is what makes Chaucer, and his two great contemporaries Langland and the Pearl-Poet, so much more interesting to modern readers than many other works: the multi-generic works are harder to "solve," less determinate in meaning, closer to what we moderns expect, than much other medieval writing. It's not until Shakespeare that we see this much depth again in one writer.