Userboxes are small colored boxes designed to appear on user pages. They generally include one or two small graphics and a few short lines of text. Often they also include wiki markup that places the user in a category. Userboxes are generally created as templates and transcluded onto userpages, although their markup may be substituted directly onto userpages instead. Userboxes should not be used in articles.
More detailed information on how to create and use Userboxes can be found on Wikipedia