What do version numbers mean?
Drupal release versions
Starting with Drupal 5.0, the "5" indicates the major version of Drupal, and the .0 is the minor version i.e. the bug-fix release or patch level. That means that 5.0, 5.1, 5.2 and so on all have the same underlying structure. Collectively, these versions are often referred to as "5.x". Modules for 5.x are generally all compatible with each other and with any Drupal 5.x version of core. However, modules written for Drupal 6.x or 7.x will not work with 5.x and vice-versa.