A genre is a type of artistic arrangement that is defined by a set of specific characteristics, formal qualities, and subject matter. A work of art is considered to be of a certain genre if it meets all the requirements presented by it.

A genre is a key concept in music, literary sciences, and visual arts. In literature, this device is represented by a group of literary works that were written using the same formal guidelines and have similar content.

The term itself comes from France and can be translated as "kind" or "sort." The first significant genre classification was done by Plato but was later changed by Aristotle. The Greek philosopher differentiated four genres: tragedy and comedy (which became parts of drama), epic (which is now considered to be a genre of poetry), and parody (which ultimately turned into a writing technique).

Genre is oftentimes confused with format. However, the latter refers solely to the physical aspect of a work, as it can be created either in a graphic-novel format or decorated with pictures and illustrations, but that doesn't relate to the content of the piece itself.

This tool is commonly mistaken with a writing style, which is an individual expression of thoughts and ideas through the means of choosing particular words and sentence and paragraph structures that suit a particular author the best. 

This device also doesn't have anything to do with age restriction, as a work of any type can be written either for adults, young adults, or kids.

The primary purpose of classifying literary pieces into separate genres is to warn the reader of their content. Even though it's hard to differentiate one genre from another, they help readers form appropriate expectations in regards to what they are about to read.

This tool also helps professors and students to better analyze the heritage of specific movements and time periods in literature. This function plays an important role both in fictional and non-fictional works.

In a narrow sense, the term "genre" refers to the formal structure of literary pieces and is separated into the following major forms: poetry, drama, and prose, with the latter traditionally separated into fiction and non-fiction works.

Poetry is usually used to convey the individual emotional state and mood of an author. This genre is characterized by the use of significant amounts of metaphors and other literary tools as well as being written in verse and possessing such qualities as rhythm and meter. Poetry is divided into specific forms such as sonnets, elegies, limericks, haikus, etc. The most notable authors that worked in this type are Lord Byron, Edgar Allen Poe, Emily Dickinson, Pablo Neruda, John Keats and numerous others.

Works written in the drama genre simultaneously belong to two forms of art – theatre and literature, as they are created to be played on stage but originally exist and are distributed in a text form. From a formal point of view, this genre is defined by the fact that it is composed solely out of dialogues, monologues, and short remarks that give context to a scene or describe the decorations. Drama consists of comedic, tragic, melodramatic works, and of different mixtures between them. Poignant examples include the plays by William Shakespeare, Henrik Ibsen, Anton Chekhov, Arthur Miller, and Tennessee Williams.

Unlike the rest of major genres, non-fiction works aren't based on the author's fantasy but rather on real facts and reflect a certain aspect of the world in an objective manner. Non-fictional genres include documentaries, autobiographies, manifests, essays, journalistic pieces, memoirs, etc.

Fiction literary pieces are created using the writer's imagination and describe events that never happened, usually, in a believable manner. This genre is the most popular one in the world and can be further classified into novels, short stories, novellas, and novelettes based on the length of the work. Authors that have fully uncovered the true potential of this genre include Charles Dickens, Ernest Hemingway, James Joyce, Thomas Mann, etc.

From a broader point of view, literary pieces are also divided into genres based on their subject matter. Thus, fictional works can be classified into horror, suspense, mystery, fantasy, humor, science fiction, gothic, adventure, and numerous other types.
Thematically, most groups are also divided into subgenres, based on the specific characteristics that are shared by a significant portion of works written in that genre. For instance, dark fantasy stories are signified by a grimmer, more depressive tone and typically depict dystopian worlds. A prime example of this subgenre is "A Song of Ice and Fire" by G.R.R. Martin.

Some genres demand that the author strictly follows the accepted rules, while others allow more room for experimentation. This is especially true for fictional works, where writers can freely switch between genres during the creation process. For instance, the "Flowers for Algernon" by Daniel Keyes has started out as a short story but later on has grown into a full-sized novel.