Drupal 7.14 is now available, which contains bug fixes as well as fixes for security vulnerabilities
Drupal 6.26, which fixes known bugs (no security issues) is also available for download.
Upgrading your existing Drupal 7 and 6 sites is strongly recommended. There are no new features in these releases.
Drupal 7.13 only includes fixes for security issues. Drupal 7.14 also includes bugfixes. The full list of changes between the 7.12 and 7.14 releases can be found by reading the 7.14 release notes.
Drupal 6.26 only includes bugfixes
Drupal 7.13 and 7.14 were released in response to the discovery of security vulnerabilities. Details can be found in the official security advisory:
To fix the security problem, please upgrade Drupal.
What is included with each release?
The Drupal team made two versions of Drupal 7 available, so you can choose to only include security fixes (Drupal 7.13) or security fixes and bugfixes (Drupal 7.14). You can choose your preferred version. The Drupal team is trying to make it easier and quicker to roll out security updates by making security-only releases available as well as ones with bugfixes included.