You may wish to restrict a page to only be visible to particular usergroups. For example, you may have a custom template page which you only want registered members to be able to view. This can be done very simply by placing a comment like this at the very top of the template:

<!-- RESTRICT TO GROUPS 2,3 -->

Change 2,3 to the numbers of the usergroups you wish to allow to view the page. Separate each usergroup number with a comma, no spaces. By default, of course, pages are visible to all groups (except where usergroup permissions specify otherwise).

For a category, you can restrict a usergroup from having access simply by editing the category.