Including the contents of one page into a host page is called transclusion. This allows the host page to automatically be updated whenever the template page is updated, and is accomplished on Wikipedia by surrounding the page name with double curly brackets and leaving out the "Template:" prefix. In addition, pages in the following namespaces can 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 also can be transcluded, but this is done using double square brackets instead of curlies. Here are some examples:
{{Pic of the day}} • {{Help:Contents}} • [[Image:Wikipedia-logo.png]]
Read more: Wikipedia:Transclusion
Wikipedia:Templates