Help:Transclusion
Transclusion is the process of include the content of a page in another page, by reference rather than by copying and pasting it. Templates are usually transcluded into pages : the template text exists once, and is editable in a single place. It is added to pages where it appears when the page is loaded.
You can transclude any page on the wiki, like so:
{{welcome}} will transclude the template [[Template:Welcome]] where it is placed in the page {{:welcome}} will transclude the page [[Welcome]] where it is placed
For more, see m:Help:Transclusion on the Meta-Wiki.
Templates
Templates are wiki pages in the Template: namespace. They are designed specifically to be transcluded. A template can be transcluded in a shorthand way, rather than saying
{{:Template:welcome}}
you can simply say
{{welcome}}
.
Common examples of templates are navigation bars for a set of pages and display templates with template variables that ensure a set of hundreds of entries are all displayed in the same style.