Topic maps are a way of modelling a set of subjects; the relationships between them; and additional information about them. The topic map paradigm is described in the ISO standard ISO 13250:2002 along with an interchange syntax which uses SGML. Since the publication of the first edition of the ISO specification early in 2000, a separate consortium, TopicMaps.Org, have produced a version of the topic map paradigm for use in a web environment which provides an interchange syntax in XML. This XML interchange syntax, known as XTM has since been adopted by ISO and forms part of the ISO 13250 standard in its second edition.
In essence a topic map can be thought of as consisting of just three things:
Describe a subject in the real world. A subject may be something that is addressable by a machine (e.g. a web-page with a URL); something which is not addressable (such as a person) or even an abstract concept (such as 'Music').
Describe a relationship between a set of topics. The association construct may include additional typing information which specifies the nature of the relationship between the topics and also what role each topic plays in the relationship. So an association can be used to represent such common concepts as a hierarchy, or a group but also more complex relationships such as a meeting (with topics representing people and playing roles such as 'chairman', 'scribe' and 'attendee').
Connect a topic to some information resource related to the topic. The occurrence can be used to specify both the information resource (or the information itself) and the nature of the relationship between the information resource and the topic. For example, a topic could represent a person and that topic could have an occurrence which is a URL which points to the 'homepage' of the person.
In addition to defining these basic structures, the topic map standards also define the way in which two or more topic maps may be combined or merged. Topic map merging is a fundamental and highly useful characteristic of topic maps as it allows a modular approach to the creation of topic maps and it provides a means for different users to share and combine their topic maps in a controlled manner.
For more in-depth coverage of the key concepts of topic maps, please see the Practical Introduction to Topic Maps. A number of other papers on the subject of topic maps can be found under the Technology section of this site.