Categorization means grouping of related pages. Articles can be sorted into different categories by tagging them with a small piece of code. Categorisation and hyperlinking are important tasks since they increase the ease of navigation and decrease the risk of redundant material.
A page may be targeted to belong to several categories. The categories of the page is shown at the bottom of the page. This page belong to the category Help and Categories because the code [[Category:Help]] and [[Category:Categories]] is added at the last line of this page which you can see if you go into edit-mode. If no code is added the page is not yet categorized. All pages should preferably be categorized in order for users to easily find related topics.
In order to watch existing categories, either see a list of all categories or check out the category tree which reflects how different categories are related - a category may namely be a subcategory to another category like the category: bioleaching microbes is a subcategory to the category:bioleaching.