Thesauri

List of preferred terms
Non-preferred terms
Broader, narrower terms
Related terms

Pros and Cons

Provide better context than simple keywords
Require special user interface

Data structure

Textual listing
XML representation
SQL statements

Metadata structure and keywords in online data catalogs
Building a National Knowledge Bank for CMGP 23-Apr-2002
Peter Schweitzer, U.S. Geological Survey