EllisLab is always working on new versions of ExpressionEngine. These versions are often packed full of new features as well as security updates and bug fixes.
Note
Occasionally, EllisLab will put a price on a new feature (such as the discussion forum). However, the prices are very fair, and reflect the cost of development and support. Doing this keeps the cost of ExpressionEngine itself lower for those users who have no interest in the new feature.
There are two types of releases that ExpressionEngine goes through:
A new build contains bug fixes or minor updates. It is generally not necessary to upgrade to a new build unless it contains a bug fix that you have been waiting for. Builds are numbered by date, and you can see what build you have in the footer of any page in the control panel.
A new version contains new features and improvements to existing functionality. A new version will include all the bug fixes and minor updates of any intermittent builds. Versions...