Categories are a vital part of all wikis, especially Wikipedia. They allow users to sort pages into groups, to allow easy and comfortable navigation. To add a page to a category, one would need to link the category at the bottom of the page. Using the example of "fruit", one would write
[[Category:Fruit]] at the bottom of the page to add the page to the category "Fruit".
Usage in templates
Templates can add categories automatically, especially if that is their function (for example: stubs). This is done by adding a category to the template. If the user does not want the template to be added to the category, they would write (let's use the example: Baseball players this time)
<includeonly>[[Category:Baseball players]]</includeonly> at the bottom of the template's code. This will make sure, when the template is used, that only pages that transclude (use) the template will be added to that category.