An XML document must be valid, well-formed, or both.
To define a set of tags for use in a particular application, XML uses a separate document named a document type definition (DTD). A DTD states what tags are allowed in an XML document and defines rules for how those tags can be used in relation to each other. It defines the elements that are allowed in the language, the attributes each element can have, and the type of information each element can hold. Documents can be verified against a DTD to ensure that they follow all the rules of the language. A document that satisfies a DTD is said to be valid.
If a document uses a DTD, the DTD must immediately follow the declaration.
XML Schema provides an alternative mechanism for describing and validating XML data. It provides a richer set of datatypes than a DTD, as well as support for namespaces, including the ability to use prefixes in instance documents and accept unknown elements and attributes from known or unknown namespaces. For more information, see the W3C XML Schema page.
The second way to specify XML syntax is to assume that a document is using its language properly. XML provides a set of generic syntax rules that must be satisfied, and as long as a document satisfies these rules, it is said to be well-formed. All valid documents must be well-formed.
Processing well-formed documents is faster than processing valid documents because the parser does not have to verify against the DTD or XML schema. When valid documents are transmitted, the DTD or XML schema must also be transmitted if the receiver does not already possess it. Well-formed documents can be sent without other information.
XML documents should conform to a DTD or XML schema if they are going to be used by more than one application. If they are not valid, there is no way to guarantee that various applications will be able to understand each other.
