Page categories are a means of "tagging" articles in the wiki based on the content of the page. This is independant of any structure imposed by project index pages, and a page may be assigned several categories it they are each relevant to some part of the material in the article.
The categories assigned to a page are shown in a grey box at the foot of the article. Clicking on one of the categories will take you to a list of all pages that share that category. This allows the user to find material of similar nature that may otherwise be difficult to find by conventional seraches or by "tunnelling" down through the wiki page structure.
To add a category to a page, insert one or more lines similar to this one
[[Category:Documentation]] near the foot of the page, which adds the "Documentation" category.
A list of the current categories in use is provided here: Special:Categories.
If you feel that none of the existing categories adequately covers some part of your article then feel free to create a new one: Simply add a category line as above, but with your new category name following the colon. Once the page has been saved, follow the link to your category and provide a description of what it is intended to include - look at some of the existing category descriptions for examples.