The XTemplate theme system uses templates to layout and style Web pages. It separates logic (PHP), structure (XHTML/HTML), and style (CSS), making it easy for designers to create or modify templates by working on XHTML/HTML and CSS without having to worry about any PHP coding.
Once a template exists in the themes directory, XTemplate auto-detects it, and makes it available for selection to administrators:
administer -> themes
Drupal is distributed with two XTemplate templates included - Bluemarine and Pushbutton.
Although XTemplate is still supported as part of the core, it may not be in the future, for several reasons. This will not necessarily mean the end of XTemplate since it may be maintained as an alternative contributed engine like PHPTemplate.