What is a plugin?
A plugin, (previously known as an add-on, contributed code, or "contrib" for short), is code enabling you to add additional features and functionality to Moodle.
A number of plugins are included in the standard Moodle download. Additional plugins may be obtained from the Moodle plugins directory.
How do I install a plugin?
See Installing plugins for instructions. Some, but not all, contributors include a "Read me" file with instructions in their plugin package.
In a typical install, you will probably copy some php files into one or more areas of you Moodle site's code, then click on the Site administration link called "Notifications" to start the install process.
How do I apply a patch?
Why doesn't my newly installed plugin show up in the list of plugins?
Firstly, check that you have visited your admin index page http://your_moodle_site.org/admin/index.php to complete the installation. Next, check that you unzipped the archive to the correct folder, and that the new plugin folder is not contained within another folder.
How do I contribute code to Moodle?
Is there any information on creating a new plugin?
Yes! See dev:Plugins.