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

What are some ways to determine if an xml document is well-formed and/or valid? (looking for websites and/or software)

>What is the difference between a well-formed XML document and a 'valid' XML document?
Well formed XML is syntactically correct (ie. a parser won't choke on it) and valid XML is semantically correct based on a provided schema.

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