Originally Posted by Horfic
plug = Plugin, a new seperate Feature
mod = Modification, Plugins modified to the needs of the customer
Nearly perfect but in this case mod means module
A module is e.g. dns, web, email. Similar to the upper navigation in the interface. Normally you dont have to add a new module as modules do not do any configuration work. Modules contain only some code to provide events.
All configuration work is done in the plugins. In your plugin you may attach as many functions to an event as you like.
To get started, the best way is to look at the existing plugins. Copy a plugin of your choice, change the file name and class name inside the plugin and create a symlink from plugins-enabled to plugins available. To see what your plugin does, you should enable debiúgging: