A Document Type Definition (DTD) is a set of rules that defines the structure and legal elements of an XML document. It specifies which elements and attributes can appear in the document, their relationships, and the data types they can contain. DTDs help ensure that XML documents are valid and conform to a specific format, making it easier to share and process data. There are two main types of DTDs: internal and external. An internal DTD is defined within the XML document itself, while an external DTD is stored in a separate file. Both types serve the same purpose of validating the structure of XML documents, ensuring consistency and accuracy in data representation.