Many things in a wiki depend on templates.
Templates are a powerful means of making stuff reusable within the wiki. They can not only serve as simple boilerplates that can be included into other articles over and over again, but also have more complex function.
There is the possibility to parameterise parts of the template. You can have "gaps" or placeholders to be filled in differently with each use of the template in another article. These placeholders or parameters are implemented by a name or number between triple curly brackets within a template.
Using a simple template is as easy as creating an internal link. You simply use curly brackets instead of square ones to surround the name of the template:
incorporates the contents of Template:Test into the calling article. See the text between the two horizontal rulers for an example:
This is the content of a test template.
There are many tricks and gimmicks you can implement using templates and their advanced features which are too complex to be dealt with in this tutorial. You can visit MediaWiki's help page on template if you are interested in learning more about them.
For a listing of all existing templates and their purpose see also Category:Template. The correct usage of a template is often explained on their respective talk pages.