'Semantics' is just the study of 'meaning'. For more information on the 'Semantic Web' here is a video of a talk given by Peter Patel-Schneider who is an expert on web ontology languages to Google staff earlier this year. Talk. - Michael
I wanted to make it clear that the Linguistic study of Semantics is quite different from the "Semantic Web" meaning of the term, which in effect is an engineering, and not academic, one. I'm curious, what would you say a semantic "rule" should be?
Peter Patel-Schneider who gave the talk above was the main editor for the semantic definition of OWL (Web Ontology Language) a markup language for publishing and sharing data using ontologies. You can see a number of example semantic rules in this paper of his A Semantic Web Rule Language Combining OWL and RuleML An over simplified example might be canine = dog OR tooth Hope that helps. - Michael