A schema defines the structure of an XML document. Schemas define elements and attributes valid for a particular namespace.

Schemas are W3C standards, and are written in XML syntax.

DITA schemas exist, but currently are seldom used. (DTDs are more common.)

The following figure shows the schema for DITA Concept.

Related concepts
DOCTYPE declaration
DTD (Document Type Definition)
XML (eXtensible Markup Language)