Validating

DTDs and XML Schema describe the valid tags, attributes, data types, etc. that can be present in a given XML document.
Validation is the process ascertaining whether a given document obeys its DTDs or Schemas.
Is should not be confused with "well-formed XML document": one that is legal XML.
Valid documents are always well-formed, but well-formed documents may not be valid.
Validation is not required when working with XML data. In fact, it is often omitted altogether (although, that is probably a bad idea).