Templates may contain any desired wikitext, including calls to other templates. They have some limited programming capacities: customizable values (called parameters), calculation and branchings (using parser functions), and access to wiki-specific variables (magic words), such as dates, times, and page names.
To use a template in an article or page, a template tag (in the form {{template name}}; the name of the template enclosed in doubled curled brackets) is added where you want the template to appear. When a reader wants to view the page, the Wikimedia servers fetch the contents of the template page, make any calculations that need to be made, and then replace the template tag in the wikitext with the newly processed contents of the template.