OFX is actually several things. At the lowest level OFX is a generic 'C' based plug-in architecture that can be used to define any kind of plug-in API. You could use this low level architecture to implement any API, however it was originally designed to host our visual effects image processing API. The basic architecture could be re-used to create other higher level APIs such as a sound effects API, a 3D API and more.
This book describes the basic OFX plug-in architecture and the visual effects plug-in API built on top of it. The visual effects API is very broad and intended to allow visual effects plug-ins to work on a wide range of host applications, including compositing hosts, rotoscopers, encoding applications, colour grading hosts, editing hosts and more I haven't thought of yet. While all these type of applications process images, they often have very different work flows and present effects to a user in incompatible ways. OFX is an attempt to deal with all of these in a clear and consistent manner.