There are three kinds of XML documents:

• Invalid documents don't follow the syntax rules defined by the XML specification. If a developer has defined rules for what the document can contain in a DTD or schema, and the document doesn't follow those rules, that document is invalid as well. (See Defining document content for a proper introduction to DTDs and schemas for XML documents.)
• Valid documents follow both the XML syntax rules and the rules defined in their DTD or schema.
• Well-formed documents follow the XML syntax rules but don't have a DTD or schema.