We may have heard the term "template" before when talking to someone about theming and Drupal. But what exactly is a template? We can think of a template as a text file no different from any HTML document that provides a method for separating the presentation layer from the business logic. In traditional PHP websites, we have the ability to mix PHP with HTML and CSS, which makes managing web pages both difficult and dangerous. Drupal provides us with the ability to use templating engines to enforce the separation of the two, so we can begin to focus more on the HTML and CSS and worry less about the PHP.
In general, templates can contain HTML markup and PHP variables that output content contained within a Drupal database. Templates can be as small as a few lines of HTML that hold the presentational layer for a block that is displayed in a region on the page, or the actual page itself, with containers defined for header, content, and so on....