The classes/plugins path contains the code that OJS and OCS use to support plugins, i.e. the plugin registry, plugin settings code, etc. The actual plugins themselves live in the plugins directory, each in a category that defines how it interacts with the system (e.g. "generic", "importexport", etc).
Have a look at the OJS Technical Reference, available from the Documentation section of the OJS web site; it has some information on OJS plugins, and OCS plugins work the same way. (Ideally we hope to eventually have a set of common plugins that work for both.)
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