ৱিকিপিডিয়া:Transclusion/How Transclusion Works
To transclude any source page (within a single MediaWiki project, such as en:Wikipedia), include the following code in the target page:
Whenever the target page A with this code is rendered, the engine will include in that place not the code itself, but the entire content of the source page B, SOMEPAGE.
For example, you might decide to place a welcome message on every newcomer's Talk Page. Transclusion creates a "live" link between the template-page and the target-page(s) upon which the message should appear. When the template is edited, all the target-pages are edited too.
For example, you create a (template) page in Wikipedia with your home mailing address and then include that template not only on your page, but all your friends' pages, too. Later, after moving to a new house, you then update your address template and the new address will automatically appear on all your friends' pages.