In Drupalese, upgrade is typically used to refer to the process of migrating to the next major release branch of Drupal, as in upgrading from Drupal 6 to Drupal 7. The related term, update, refers to the process of performing minor stability or security updates for modules and core within in the same major release branch of Drupal, as in updating from Drupal 6.19 to Drupal 6.20. This guide covers major revisions upgrades only.
These notes were compiled in the process of upgrading from Drupal 6.20 to 7.0-rc3. Links to reference resources can be found below.