The Drupal Taxonomy system
Taxonomy is the classification of things into categories and types. The most widely known example of taxonomy is the taxonomical classification of plants which works by describing several characteristics.
In Drupal taxonomy is a relatively simple categorization system. The administrator defines one or more taxonomies, and adds terms to the taxonomies. A taxonomy can also be thought of as a category. Terms can be thought of as keywords. A taxonomy (or category), then, can be thought of as a group of keywords (or terms).
The Drupal taxonomy system is very powerful and can be used for many things.