The term "theme" when used in the context of content management systems (cms), galleries, or e-commerce systems refers to the use of templating systems such as (phptemplate or smarty) which are used to structure and control the appearance of dynamically generated web content.
Drupal is the world’s most powerful, most flexible and most rapidly expanding open source content management system (CMS). The truth of the matter is that it is far more powerful than commercial options as well. Drupal can be used for building just about any kind of website you can imagine, from a simple online business card to a social networking site like Facebook. The main attraction of using CMS websites rather than flat html based websites is that a content management systems allow you to edit your pages directly on the site itself.
If you are asking this question it’s pretty safe to say that you are new to Drupal. After you’ve been dealing with Drupal for a while it’s easy to start thinking everyone knows what you are talking about when you start throwing out terms like “Drupal Modules” and “Drupal Themes”, but the truth of the matter is that to the average Joe on the street this all sounds like gibberish.