The Plugin Plugin is used to create a Maven plugin descriptor for any Mojo's found in the source tree, to include in the JAR. It is also used to generate Xdoc files for the Mojos as well as for updating the plugin registry, the artifact metadata and a generic help goal.
The Resources Plugin handles the copying of project resources to the output directory. There are two different kinds of resources: main resources and test resources. The difference is that the main resources are the resources associated to the main source code while the test resources are associated to the test source code. Thus, this allows the separation of resources for the main source code and its unit tests.