From Rosetta Code

This is a substitutable template that is used to create signatures for users. To make use of it, write this:

{{subst::User:Dkf/Signature|username|contribution datetime}}

That then causes content like this to be inserted into the page:

<div><small>''(This was written by [[User:username|username]] ([[User_talk:username|Talk]] | [[Special:Contributions/username|contribs]]) at contribution datetime)''</small></div>

Which renders like this:

(This was written by username (Talk | contribs) at contribution datetime)

This is a template. There are many others. See Category:RCTemplates for a complete list of templates.