While XNAT does not require plugins to contain any particular files (a plugin could contain only a single properties file), only plugins containing a java class with the @XnatPlugin annotation will show up in this list.
List of Installed Plugins
This list shows some basic information about your currently installed XNAT plugins.
Plugin Information Modal
For more information about the plugins, you can click the 'View Plugin Info links in the above plugins table. This opens a modal that will look something like this: