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.
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: