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DTD stands for Document Type Definition. It is a set of markup declarations that define the structure and rules for a document type, such as an XML or HTML file. A DTD specifies which elements, attributes, and entities are allowed in a document and their relationships to one another, acting as a schema to validate the document's format.

Key aspects of a DTD:

Structure and validation: A DTD defines the grammar for a document type, ensuring that the document is well-formed and consistent. This allows for consistent processing and data exchange between systems.

Elements and attributes: It specifies the names of the tags (elements) that can be used and the attributes they can have. For example, a DTD can define if an element can only contain text or child elements.

Relationships: A DTD defines the relationships between elements, such as which elements can be nested inside others. For instance, it can specify that a paragraph element is allowed inside a table element.

Examples of use: DTDs are commonly used for defining the structure of XML documents, older versions of HTML (like HTML 4.01), configuration files, RSS feeds, and data interchange formats like those used in web services.

Modern alternatives: While DTDs were foundational, for modern XML applications, the preference is often for the more powerful and flexible XML Schema (XSD), which offers more data typing options and is easier to work with.