PHPTemplate:
A theme engine that allows you to use template files written in pure
php. These template files do not need to be processed by the theme
engine, and as such execute a lot faster than most other template
engines. Another major advantage to using PHP as your template
language, is flexibility, as the advanced user can access any
information / functionality available in the Drupal API, and is not
restricted to what the template engine / language allows him to do.
This is the default theme engine starting in Drupal 4.7. If you are
using Drupal 4.7 and above, you don't need to download PHPTemplate
here.
A Template system does not need to use special code or markup to be called a template. PHPTemplate refers the system of implementing pure php code template files, how those files are accessed and referenced by the system and how other parts of the system (such as template.php) can modify the results of those files