Well, "well-formed" XML is syntactically valid XML. "Valid" XML is always well-formed, but must also be semantically correct. XML is not valid when you use unknown elements, attributes or invalid child elements etc. example: Let's say you have XML document describing car. You have some schema (.xsd file), allowing only wheel element inside car element.
<car>
<wheel />
<wheel />
<sometotalydifferentxmlelement />
</car>
is well-formed XML, but invalid.
"Throughout human history, we have been dependent on machines to survive. Fate, it seems, is not without a sense of irony. " - Morpheus