What is this Joomla/Mambo Community Builder?

Difficult to really produce a concise definition, but its all of the following:

• A Joomla/Mambo component and 3 collaborating core modules
• An environment for producing and maintaining online communities on Joomla/Mambo driven sites
• A system that provides user profile capabilities for Joomla/Mambo sites
• A different registration/login process for Joomla/Mambo sites (supports 4 different workflows)
• An environment that enables Mambo/Joomla site administrators to create user profiles that capture extra information from users (with the creation of new fields) that can be presented and organized in tabs (again defined by the administrator)
• A framework for other Joomla/Mambo components to integrate with user profiles

I could really go on ... but I think you are starting to get the picture. You can see a lot of the Community Builder capabilities by visiting your own profile on this site!