Including the contents of one page ("the template") into a host page is called transclusion. This allows the host page to automatically be updated whenever the template page is updated. It is accomplished on Wikipedia by surrounding the name of the page from which the contents are being transcluded with two pairs of curly brackets and leaving out the "Template:" prefix. In addition, pages in the following namespaces can also be transcluded: Category:, Help:, Portal:, User:, and Wikipedia:. The only difference is that you must include the prefix (followed by a colon) with the page's name inside the double curly brackets. Images can also be transcluded, but this is done using double square brackets instead of curly ones. Here are some examples of templates that will transclude if you paste them, save them, and then refresh or purge the page:

((Pic of the day)) · ((Tip of the day)) · ((Help:Contents)) · [[File:Wikipedia-logo.png]]