Drupal is basically a content management framework. But it is capable of doing more than just that. The Drupal software is equipped with many features that not only make it functional but also make the websites that use it to be aesthetically pleasing to the eye. The Drupal software has features like the Drupal themes that enable the software to do just this.
Drupal themes are essentially there to alter the look and feel of the different websites. There are several Drupal core themes as a basic feature. A website administrator can easily choose among the supplied themes through a special menu. An additional customization tool for administrators is through a web-browser interface which allows changing of color schemes in themes.
Aside from the core themes, there are also quite so many contributed themes. And they are there to replace the look and feel of the default themes in the software. These themes can be sourced through third party design engines. They are in standard Drupal format and most of them are basically written in PHP.
These themes are not really a part of the official Drupal release. Since they are made by third party theme authors, some of them might not work as expected. One thing to remember is that themes should match the Drupal version it is written for. Something written for Drupal 5.x will certainly not work in Drupal 6.x. And since Drupal welcome contributions from its users, these contributed Drupal themes are actually found on the Drupal website itself.