OFX is actually several things. At its base it is a generic plug-in architecture which can be used to implement a variety of plug-in APIs. The first such API to be implemented on the core architecture is the OFX Image Effect Plug-in API.
It is all specified using the 'C' programming language. C and C++ are the languages mainly used to write visual effects applications (the initial target for OFX plug-in APIs) and have a very wide adoption across most operating systems with many available compilers. By making the API C, rather than C++, you remove the whole set of problems around C++ symbol mangling between the host and plug-in.
APIs are defined in OFX by only a set of C header files and associated documentation. There are no binary libraries for a plug-in or host to link against.
Hosts rely on two symbols within a plug-in, all other communication is boot strapped from those two symbols. The plug-in has no symbolic dependancies from the host. This minimal symbolic dependancy allows for run-time determination of what features to provide over the API, making implementation much more flexible and less prone to backwards compatibility problems.
Plug-ins, via the two exposed symbols, indicate the API they implement, the version of the API, their name, their version and their main entry point.
A host communicates with a plug-in via sending 'actions' to the plug-in's main entry function. Actions are C strings that indicate the specific operation to be carried out. They are associated with sets of properties, which allows the main entry function to behave as a generic function.
A plug-in communicates with a host by using sets of function pointers given it by the host. These sets of function pointers, known as 'suites', are named via a C string and a version number. They are returned on request from the host as pointers within a C struct.
Properties are typed value/name pairs that exist on the various OFX objects and are action argument values to the plug-in's main entry point. They are how a plug-in and host pass individual values back and forth to each other. The property suite, defined inside ofxProperty.h is used to do this.