Econtnent Mag:

"A simple example of a topic map is illustrated in a "gentle" introduction to the subject on Topicmaps.org. A topic map discussing Shakespeare might include some explicit links to URLs for some of the plays, connecting the URLs to "topics" [...]. Each of these URLs is considered an "occurrence" of each topic. These two occurrences can then be tied together with an "association"; this seemingly simple connection is what begins to give topic maps their power. Taken together, these topics, occurrences, and associations begin to form a topic map."