What is the difference between an ontology (OWL) and a schema (RDFS)?
In essence, nothing. Both are formal specifications of some conceptualisation or abstract world model.
From its usage in the Semantic Web community and the broader knowledge-based systems community, the term 'schema' is used to refer to simple conceptualisations, as might be written using less expressive knowledge representation languages such as RDF Schema (so making explicit the relationship between ontologies and database schemas). Conversely, the term 'ontology' is taken to describe more complex models, written in more expressive languages (OWL, DAML+OIL, Ontolingua, KIF, etc).
There is a school of thought that considers ontologies to contain rule-based knowledge in addition to a relational characterisation, but this is far less prevalent in the SW community than elsewhere.
Other related terms for degenerate ontologies include taxonomy, which refers to ontologies that include only a class hierarchy (commonly based on the properties broader term/narrower term/related term), and meronomy (a part-of containment hierarchy).
adapted from nmg to rdfweb-dev 29 Sep 2003