It’s not always easy to understand Drupal’s version numbering scheme for the Drupal core and contributed modules. This article will try to help you to understand what the versions numbers mean and what the status is of each current version.
Two major versions of Drupal are always consistently supported at all times. Right now, the two supported versions are Drupal 6 and Drupal 7. To support evolving code, updated versions of Drupal are released regularly for each of these: 6.0, 6.1, 6.2, etc. These versions can be collectively referred to as “6.x,” and eventually “7.x.” The newest version in each series fixes problems that were discovered from the previous version, and these fixes can include critical security updates. The best practice is to always run the most current version of the series, no matter which version you are using for your Drupal development project.
Several determinants should be taken into consideration when making the decision to use Drupal 6 and 7 for your Drupal development project. Drupal 7 is the newer version with the most up-to-date features and enhancements, and will be supported longer than Drupal 6. However, the functionality you need to enhance your site may not yet be available for Drupal 7. The releases of contributed modules, themes, translations, and installation profiles compatible with each major Drupal release series sometimes lag behind Drupal itself so this could have a big impact on your decision for your Drupal development project.
Currently, Drupal 8 is in the early stages of development and is not yet ready for release. The stable version of Drupal 8.0 is slated to be released in August 2013. Once Drupal 8 is released, Drupal 6 will no longer be supported or receive security updates, and you will need to upgrade your Drupal 6 sites to Drupal 7 at that time. Compatibility with modules, themes, translations and installation profiles will also begin to flourish.