We can distinguish three kinds of concepts in RDF: fundamental concepts, schema-definition concepts (useful for defining new vocabularies) and utility concepts (concepts which are not absolutely necessary, but likely to be useful in any application domain).
All these concepts have been given a URI. These URIs are defined as fragment
identifiers of the URIs of the W3C documents defining RDF. For the sake of
clarity, we will rather use the XML non-expanded notation ; that is, prefixes
rdf: and rdfs: will be used instead of
http://www.w3.org/TR/1999/REC-rdf-syntax-19990222# and
http://www.w3.org/TR/1999/PR-rdf-schema-19990303# respectively.
The membership of one or the other namespace may not always seem logical, and
must have historical reasons mostly.