As nouns the difference between law and sanction is that law is (uncountable) the body of rules and standards issued by a government, or to be applied by courts and similar authorities or law can be (obsolete) a tumulus of stones while sanction is an approval, by an authority, generally one that makes something valid.
As a interjection law is (dated) an exclamation of mild surprise; lawks.
As a verb sanction is to ratify; to make valid.