Plugins are extensions to Stellarium. They are potentially more powerful than scripts, but are more difficult to write and maintain. Unlike scripts, plugins must be compiled for a specific platform, and will typically only be compatible with a particular version of Stellarium.
We hope that the plugin system will allow third party developers to write extensions to Stellarium which might not otherwise be included in the core program, and that the system will allow for prototyping of new features before inclusion into the core.
Details for plugins architecture and types you can find in Stellarium Developers Documentation.