Joomlatools

1.4, 1.5, 1.6... are feature releases. They add new things, and they are mostly backwards compatible. This means that upgrading from one feature release to the next will be fairly easy, and 3PD addons such as themes, translations, utilities, ... will work with little to no modifications.
1.5.0, 1.5.1, 1.5.2... are maintenance releases. They consist only of bug fixes and security fixes, are 100% backwards compatible.