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.
Not all information has been updated for Drupal 7, but more information is continually being added about Drupal 7 theming.