Plugins are scripts which you create, allowing you to let your Users to
do tasks, but only with the process level of their own user ID*.
The main benefit of plugins is their ability to be installed with 1 click, work on any OS, any Skin, without needing to do any other modifications to your setup. They're also "safe" in the sense that the calls made by a plugin run only as the User, and not root.
In computing, a plug-in is a set of software components that adds specific capabilities to a larger software application. If supported, plug-ins enable customizing the functionality of an application. For example, plug-ins are commonly used in web browsers to play video, scan for viruses, and display new file types. Well-known plug-ins examples are the Adobe Flash Player, Adobe Acrobat, QuickTime and Microsoft Office 2007.
*You can get root access with a plugin if you create a plugin binary with chmod 4755, but can be a huge security risk if done incorrectly, so not usually recommended unless you're completely confident in your security-coding abilities.