This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
A well formed xml is one that is structurally valid. There are many points that dictate the well formedness. Here is a checklist and this is an article on the same.
A valid xml is one that is well formed and additionally also follow a schema/dtd associated with the xml instance. An xml instance is valid against a schema/dtd, i.e. if you say "this xml instance is valid", it will not be a complete statement. You must say that "this xml instance is valid for this schema/dtd"