is a branch of metaphysics
concerned with the study of knowledge
or what is. An ontology
is a theory
or set of theories about knowledge and reality.
About the two most commonplace uses of ontology are in curriculum selection (deciding what should and shouldn't be taught in schools) and classification systems (such as the Library of Congress Classification System). These are encodings of what knowledge is worth passing on and what knowledge exists, respectively.
Because of the close historical relationship between libraries and scholarship classification systems have traditionally been influenced by the curriculum of universities and colleges.