The functionality of GIMP can be enhanced by adding plugins. There are hundreds of plugins available or you can create your own. GIMP plugins (also known as scripts) can be created using two different programming languages. Python or Script. These two systems are also known as "Python-fu" and "Script-fu" within the GIMP community. Python plugins have the .py extensions and Script plugins have a .scm extension.

If you have downloaded a plugin from the internet (or the GIMP Plugin Registry) it may be compressed into an archive (ZIP,RAR,GZ etc) and must be extracted first. I use the free 7Zip utility for this. Once extracted you need to copy into the appropriate GIMP folder.

The standard locations are shown below :

Python-fu Plugins
C:\Program Files\GIMP 2\lib\gimp\X.X\plug-ins

Script-fu Plugins
C:\Program Files\GIMP 2\share\gimp\X.X\scripts

where X.X is a version number. At the time of writing this is currently "2.0".

If you can't find the directories you can search your harddrive for "*.scm" or "gimp*.py" and this will reveal the location of existing .scm and .py scripts.

Once you've copied the files into the correct directory you can launch GIMP.

Where is my plugin?
If everything has gone to plan the script should be referenced in the main drop down menu under "Filters". The exact location is specified in the Plugin file and is something the author would have decided at the time the script was written. Plugins can also be placed under the other main drop down menus such as "Layers".

Summary:
So the process is :

- Obtain Python or Script plugin file(s)
- Copy to "\lib\gimp\X.X\plug-ins" or "\share\gimp\X.X\scripts"
- Launch GIMP

Changes to the plugin directories are only picked up when GIMP starts so if you have added new Plugins make sure you re-start GIMP otherwise it will not notice them.

Linux
If you are using a flavour of Linux, such as Ubuntu, the GIMP Plugin Installation page on Wikibooks discusses the details.