Templating - Drupal's theme system separates content from presentation allowing you to control the look and feel of your Drupal site. Templates are created from standard HTML and PHP coding meaning that you don't have to learn a proprietary templating language.
Detailed information links to --> How to write themable modules: http://drupal.org/node/306
Blocks Regions Variables available in theme template Additonal variables in theme template Customizable templates Customizing themes for node types, terms, sections, paths, and front page Theme functions
What else should we describe?
Kieran