I would first like to preface this with a warning. When you start talking about validity of a contract, you are straying toward the practice of law. If anyone ever asks you if their contract is Valid, refer them to an attorney. Now for the definitions.
Valid - a valid contract meets all the legal requirements for validity (competent parties, mutual consent, valuable consideration, legal purpose, voluntary act of good faith). A Valid contract is generally enforceable but there are a few exceptions.
Void - a void contract is missing one (or more) of the legal requirements for validity. A Void contract is unenforceable.