Document Type Definition
XML markup declarations are bundled together into documents called "DTDs" (Document Type Definitions).
DTDs were developed as part of ISO SGML (IS 8879:1986) and provide very simple and terse grammars, simple data-types enough for document-local links and symbolic processing, and the entity mechanism for linking to external resources.
DTDs provide a good 80/20 solution for basic QA on documents. Do not worry too much if your DTD must be more permissive than you like: DTDs only have limited modeling capabilities, and with the Topologi Schematron Validator, you can use DTDs, W3C XML Schemas schema and Schematron schemas all at the same time, if you need to.
For the grammar of DTDs, see the XML 1.0 Recommendation at W3C.