Concept mapping is a technique for representing knowledge in graphs. Knowledge graphs are networks of concepts. Networks consist of nodes (points/vertices) and links (arcs/edges). Nodes represent concepts and links represent the relations between concepts.
Concepts, and sometimes links, are labeled. Links can be non-, uni- or bi-directional. Concepts and links may be categorized, they can be simply associative, specified or divided in categories such as causal or temporal relations.
Concept mapping can be done for for several purposes:
to generate ideas (brain storming, etc.);
to design a complex structure (long texts, hypermedia, large web sites, etc.);
to communicate complex ideas;
to aid learning by explicitly integrating new and old knowledge;
to assess understanding or diagnose misunderstanding.
The concept mapping technique was developed by Prof. Joseph D. Novak at Cornell University in the 1960s. This work was based on the theories of David Ausubel, who stressed the importance of prior knowledge in being able to learn about new concepts. Novak concluded that "Meaningful learning involves the assimilation of new concepts and propositions into existing cognitive structures".
Visit the Concept Mapping Homepage at users.edte.utwente.nl 
The above is intentionally out of date as a memorial to its author. DMOZ (see Open Directory Project) has a category, though:
An example: www.luca.demon.co.uk from Theory Of Objects
See Mind Map, Novaks Writings
For a really excellent example of a tool aimed at creating concept maps, cf. cmap.ihmc.us (W32/Linux/Solaris; freeware); it is still being developed and is updated regularly.
With the Easy-Mapping-Tool you can draw maps online, you only need a browser (basic online version is free, advanced version is commercial, (UNIX/Linux/Windows/Mac Osx): www.cognitive-tools.com - online available mapping-software for learning and thinking
A random visitor claimed: "The premier concept mapping tool used in K-12 education is Inspiration by Inspiration Software: inspiration.com " Unless "Inspiration" really does occupy a leading role in the North American K-12 education field, I suggest this paragraph be deleted as it sounds like blatant advertising.
A visualization:
www.nwlink.com
See original on c2.com