A plugin is exactly what it sounds like, something that you plug in to a Minecraft Bukkit server in order to add more functionality to it. Plugins come in all shapes and sizes, some are made to make the game more enjoyable, others are made to add extra functions that server staff member can use to moderate their server.
How they are madeEdit
Bukkit plugins are usually developed in the Java programming language using an IDE such as Eclipse. Most plugins come with a permission node which limits the functionality of that plugin to a certain group of people.