It's a writing style and format for academic documents such as scholarly journal articles and books. It is commonly used for citing sources within the field of behavioral and social sciences, including sociology, education, health sciences, criminal justice, and anthropology, as well as psychology. 

It provides guidelines for writing and documentation of research in the humanities, such as English studies (including the English language, writing, and literature written in English); the study of other modern languages and literatures, including comparative literature; literary criticism; media studies; cultural studies; and related disciplines.

This citation style was created by the University of Chicago. It is commonly used for citing sources in History and occasionally in the Humanities, Sciences, and Social Sciences.