YaBB is one of the oldest Forum Software Frameworks around, YaBB was born in 2000! YaBB's Strength has always been its flexibility.
Before we get into details, you will notice I used the term 'frameworks'. This is because while the YaBB team worked diligently to make YaBB extensible and still make it accessible to non-professional developers, webmasters and hobbyists, they built in all the infrastructure to 'make YaBB your own'. So although YaBB is a pioneer forum, internally it has many of the hallmarks of a framework (although not an Object Oriented framework).