From Textpattern CMS User Documentation
Textpattern tags are arguably the most interesting and important feature of Textpattern. They represent a shorthand method for calling functions defined inside Textpattern core. The functions themselves represent content components, their sources, and means of displaying them. The tags may be added to Pages and Forms in a great variety of ways depending on what you are trying to do. The tags are replaced with the content output they call as users move from one page to another in your site.
Below are a series of tag fundamentals articles, which will get you up and running with Textpattern tags. To learn precise details about the tags themselves, see the Tag Reference.