This document discusses Document Type Definitions (DTDs) which define the legal building blocks of an XML document including elements and attributes. It explains that a DTD can be declared inline or externally. The key purposes of a DTD are to allow data interchange between groups using a standard definition, and to validate received and internal data. The document defines XML building blocks like PCDATA and CDATA, and how to declare elements as empty, containing parsed character data, having any contents, or containing child elements with defined occurrences. It also distinguishes between internal and external DTD declarations.