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Plugins are the unit of installation. They can be revised out of lockstep from the IdP. The sole task of a plugin is to inject jarfiles into the IdP's webapp in a controlled way. To ensure security, plugin distributions have to be signed. Plugin development is described here.
Modules
Modules are a unit of configuration. Modules can be shipped as part of the IdP or inside a plugin; indeed, plugins will usually (but not always) include at least one Module. Module development is described here.