Syntax is how information is represented. Semantics is what information means. Semantics are sometimes borrowed from the natural language. Semantics are sometimes inherent to the structure. Semantics are sometimes derived from logic.
Schemas describe a particular syntax, as well as the semantics of that syntax's structure.
The more syntax and semantics an agent can understand without human intervention, the more powerful and flexible the agent becomes. Structures written in XML and extends capabilities of DTDs. Canonical XML is a consistent way of expressing an Information Set. XML-signatures are generated from a hash over a list of references and the digest value of the references' content.