Wikipedia:Tip of the day/September 24, 2007
There are two main ways to use templates on articles: transclusion (accomplished by using {{Template Name}}), and inclusion ({{subst:Template Name}}). The former will include the content of Template Name on the fly whenever the article is loaded, while the latter will permanently insert the content of the template into the article. Thus, using inclusion (also called "substitution"), if the template content is modified at a later date, the article's content will not change.
Inclusion is the preferred method for long-term, permanent notices because it is less confusing, and it even helps to lighten the load on the database. Inclusion has a further advantage in that a template's content may be de-linked from any associated category or slightly modified to suit the circumstances, such as when the template is used on a talk page. Transclusion is preferred when it is possible that the template will be edited or replaced at a later date.
Read more: Templates: what they are and how to use them