Taxonomy is the word used by Drupalistas (and biologists) when talking about categorizing. The tags used on articles in a standard Drupal installation are an example of how Drupal's taxonomy system can be used.
Categories, such as individual tags on articles, are collected in vocabularies, the Drupal-Speek term for a collection of tags. A vocabulary called cities could contain categories like Miami or Mumbai, while a vocabulary called news sections could contain categories like sports, science and politics. In a standard installation, the article tags are collected in a vocabulary simply called tags.
Since a good Drupal developer needs to know Drupal-speak well, you should also know that categories in Drupal are technically referred to as terms.
It may take a little while before you use these words without getting confused, but it helps knowing them when reading or talking about Drupal. Here is a quick summary for your phrase book:
- Taxonomy is the framework for categorizing – the practice of classifying things.
- A vocabulary is a set of categories, used to separate different kinds of categories. An example vocabulary could be cities.
- Terms are the actual category words, such as Miami or Mumbai. Each term belongs to exactly one vocabulary.
TIP: You will often see the phrase taxonomy term, used to make clear that we're dealing with terms in the Drupal sense, not terms in general. Sometimes the word taxonomy is used as a synonym to vocabulary, which is technically wrong but correct in a more general sense.