2.5. Plug-ins

In the GIMP, effects and filters are implemented using plug-ins.
Plug-ins are separate programs that communicate with the main GIMP program using pipes.
Most of the plug-ins that are distributed with the GIMP are GPLed, but it is legal and possible to write proprietary plugins.
Except for plug-ins written in C, GIMP has extensions for writing filters and scripts in Scheme, Perl, Python and other languages.

Create an image, and draw some sharp marks on it using the pencil and paintbrush.
Invoke <Image> → Filters → Blur → Blur
The image is now blurred.
You can repeat the Blur by pressing Ctrl+F.