Community Builder, or "CB," is the name of a very popular, mostly free, Joomla! extension that has been available to add-on to a "free open source" Joomla! web site for many years. CB has been steadily improved by it's team of developers and it integrates (plays well) with many other Joomla! extensions. The owners of "a better developed and provisioned (more feature-rich) CB-using Joomla! site," a "Pro CB" Joomla! site, will pay $100 or $200 or more every year or two for the privilege.
The more modern, powerful, but much more complex system with similar to very much-better, yet entirely free features is CiviCRM.