There were several changes in the theme system between Drupal 6 and 7. This guide contains information on both.
The default theme for Drupal 7 is Bartik, and for Drupal 6 is Garland.
For a collection of useful materials for themers, see Theming and Front End Development with Drupal.
Please note: Not all information has been updated for Drupal 7, but more information is continually being added about Drupal 7 theming.