According to Wikipedia :
Document Type Definition (DTD) is a set of markup declarations that define a document type for SGML-family markup languages (SGML, XML, HTML). DTDs were a precursor to XML schemas and have a similar function, although different capabilities.
DTDs use a terse formal syntax that declares precisely which elements and references may appear where in the document of the particular type, and what the elementsí contents and attributes are. DTDs also declare entities which may be used in the instance document.