A Topic Map consists of Topics?, which are linked together by Associations?. Topics also are linked to Occurrences? (of Topics). Occurrences might be web pages that mention the Topic. If the Occurrences are regarded as Resources? with a relationship of "aboutness" to the Topic Resource, then Topic Maps are just special cases of RDF information structures.
Recently, Topic Maps have been reformulated to be written in XML. XTM? is XML for Topic Maps.