Wikipedia:Category types

The basic idea is to label categories so that people understand the different ways we use categories. The language is written for the users of wikipedia to help explain the use of the category, while at the same time it indirectly helps editors know how the category should be populated. There are templates for each of the following:

Topic Categories
Topic categories are high level categories containing both articles and subcategories. Typically, these do not contain any articles about specific instances of the topic. An example of this type of category is Category:Film. This category has articles and subcategories about the topic of film, but does not have any articles about individual film productions. Topic categories can be labeled with  shown below:

Index Categories
Index Categories are the "Primary" category talked about at WP:CI. An index category contains all the articles that are members of a class of articles defined by a topic. It is a master index of the topic. All members of one of these "X" categories are an "X". An example of this type is Category:Suspension bridges. Index categories can be labeled with  shown below:

Sub-index categories
Sub-index categories are secondary index categories, a more specific means of browsing through smaller groups of articles. Articles put in sub-indexes could also be put in the index categories that are their parents (there is not yet broad consensus about this). An example of this type is Category:Bridges in New York City. Sub-index categories can be labeled with  shown below:

Intersection categories
These categories are intersection of two (or perhaps three) other index categories. The articles placed in these categories should also usually be in the parent categories. This is often not currently the case in English Wikipedia. The template provides links to the parent categories. An example of this type is Category:American silent films. Intersection categories can be labeled with  shown below:

Here is another example for Category:Toll bridges in New York City:

Navigation categories
These categories only contain subcategories and are intended to help people navigate through the category structure. An example of this is Category:Films by type. Navigation categories can be labeled with  shown below:

Subject categories
These are low level categories mainly containing articles that are related to a single subject. Categories like this help users find articles that include a major section about the subject. Typically, they are eponymous categories and not part of any larger taxonomy. Subjects that are mentioned in just a few articles usually do not benefit by having subject categories. An example of a subject category is Category:George W. Bush. Subject categories can be labeled with  shown below:

[[Category
 * Wikipedia categorization]]