It is the body of literature describing the lives and veneration of the Christian saints. The literature of hagiography embraces acts of the martyrs (i.e., accounts of their trials and deaths); biographies of saintly monks, bishops, princes, or virgins; and accounts of miracles connected with saints' tombs, relics, icons, or statues.
Hagiographies have been written from the 2nd century AD to instruct and edify readers and glorify the saints. In the Middle Ages it was customary to read aloud at divine office and in the monastic refectory (dining hall) biographies of the principal saints on their feast days. Besides biographies of single saints, other works of hagiography told the stories of a class of saints, such as Eusebius of Caesarea's account of the martyrs of Palestine (4th century AD) and Pope Gregory I the Great's Dialogues, a collection of stories about Saint Benedict and other 6th-century Latin monks. Perhaps the most important hagiographic collection is the Legenda aurea (Golden Legend) of Jacobus de Voragine in the 13th century. Modern critical hagiography began in 17th-century Flanders with the Jesuit ecclesiastic Jean Bolland and his successors, who became known as Bollandists.
The importance of hagiography derives from the vital role that the veneration of the saints played throughout medieval civilization in both eastern and western Christendom. Second, this literature preserves much valuable information not only about religious beliefs and customs but also about daily life, institutions, and events in historical periods for which other evidence is either imprecise or nonexistent.
The hagiographer has a threefold task: to collect all the material relevant to each particular saint, to edit the documents according to the best methods of textual criticism, and to interpret the evidence by using literary, historical, and any other pertinent criteria.
Source: Encyclopaedia Britannica.