# The vocabularies and models (Karen's rewrite)

Metadata standards define **vocabularies** for use in instance data. Standards are often based on **models**, like entity-relationship models, of the domain being described. Sometimes the combination of a vocabulary and a model is called a schema, however the term schema is used in very different ways in different metadata frameworks.

The vocabulary comprises descriptive terms and may define relationships between the terms. It may include concept schemes that are used with the metadata standard. Concept schemes may be taxonomic in nature, like a subject classification, but they may also be simple lists.

A metadata standard can have usage rules, and these may be an actionable component of the metadata standard or accompany the standard in documentation.

A metadata standard using RDF will define **properties** and **classes** that are identified with IRIs.

A **property** is a predicate that may be used in instance data, that is, it defines a characteristic or relationship that may be asserted. A **class** identifies a type of entity or an entity group. (?)

A schema may provide rules about which properties may used to describe entities of which classes, and what data types or classes should be used as the object of those properties. Sometimes this is called an ontology, but the difference between schema and ontology need not concern us here.

Where other data formats are used for the metadata definition, the general composition will be similar, with a defined **model**, a **vocabulary** of terms, and some rules regarding structures and values.

###### tags: `dctap`