What is the difference between well-formed XML and valid XML?

Well-formed XML:

• Conforms to XML syntax rules (e.g., all tags are closed, properly nested, only one root element).
• The parser can successfully process the document.

Valid XML:

• Is well-formed XML.
• Conforms to the rules defined in the associated DTD or XML Schema.
• Is checked for compliance with data structure, element types, and attributes.

Example:

<!-- well-formed, but not necessarily valid -->
<item>Title</item>

<!-- well-formed and valid (if DTD/Schema define the 'item' element with an 'id' attribute of type integer) -->
<item id="123">Another title</item>