In the words of S&P:

* Script-Fu is the first GIMP scripting extension. Extensions are separate processes that communicate with the GIMP in the same way that plug-ins do. The distinction is that extensions don't require an active image to operate on, instead extending GIMP's functionality. GIMP internals for version 1.0 have been completely overhauled from version 0.54. In particular, the plug-in API has been made far more general with the advent of the procedural database (PDB). The PDB allows GIMP and its plug-ins to register procedures which can then be called from anywhere: internally, from extensions, and from plug-ins. There are already over 200 internal GIMP procedures, and more being created all the time. Because all of these procedures can be easily invoked from extensions, the logical next step was to create a scripting facility; thus, Script-Fu was born.