You can also extend GIMP by installing optional plugins and scripts. Dozens of scripts and plugins are installed with the default installation, but many more can be found at the GIMP Plugin Registry and at Deviant Art.
GIMP has its own scripting language, called Scheme, that is used to create Script-Fu scripts. You can identify a script-fu script by the .scm file extension. These scripts can be moved to the scripts directory found at ~/.gimp-2.6/scripts.
Plugins are written in C, Python or Perl. The plugins must be installed to be used. Often installing the plugin simply means uncompressing it and moving it to the ~/.gimp-2.6/plugins directory. However, there are some plugins that have to be compiled before use. Always read the accompanying documentation for the correct installation procedure.