Drupal comes with a built in taxonomy system that allows for categorization of the nodes on a site. The taxonomy system allows for arbitrary definition of terms, as well as arbitrary organization of those terms into vocabularies. There is no limit to the number of vocabularies that can be created, nor is there any limit to the number of terms that can be included in a vocabulary. A vocabulary can also have free tagging which means that, instead of entering specific terms ahead of time, users may enter tags freely at the time the content is created and those tags automatically become terms in that vocabulary. Drupal’s taxonomy system is one if its most powerful and flexible features.