Shelter.nu has a good writeup on what topic maps are:

We keep trying to create technological solutions that resemble human nature so that the information can be processed and handled as good as possible by both man and machine. Sometimes the abstraction happens in the user interface, where we create a cute icon for a complex computation or write the word "do" when what really happens is "do, fiddle, tweak, load, count, compute, save, tweak, squiggle, save again, spit and you're done." Other times the abstraction happens on a data model level, creating tables in such a way as to make human sense. Maybe the abstraction is on a hardware level. And, in fact, bits of abstractions are everywhere, from the inner CPU out through software to the keyboard you type on and the screen you're viewing. Unfortunately, all these bits of abstractions don't necessary make it easy to grasp what is going on, because they are - surprise, surprise! - bits that more often than not speak their own parables, and don't form a complete story.

Topic Maps is an abstraction that tries to bring together quite a lot of these bits, from the data model to the user interface, making an effort to try to tell the same story across the many layers we have in computers. And as such, it not only permutes through the technical layers of "data model" and "user interface", but also the people involved in using it, from designers and developers, project managers and general management, to users and interested parties. John the developer can now speak in the same language as the user, which is no small feat in itself and one that should lower the cost of miscommunication.

...via elearningpost