A well-formed document is balanced and syntactically correct.

It isn't necessary that all the "parts" to a well-formed document were found (e.g. external references, DTDs, etc.)

A valid document has had all its parts verified and conforms to a DTD.

Validating can mark ignorable whitespace and default attribute values.

In contrast, schemas have their own definition "validity" since their process comes after validation by an XML processor.