The core is pcb-rnd's core source code that lives in pcb-rnd svn, in trunk/src. Maintained by the pcb-rnd team.
A plugin is a small portion of the code.
Core plugins are those plugins that are in pcb-rnd svn, in trunk/src_plugins. Maintained by the pcb-rnd team.
External plugins are those plugins that are written, maintained and hosted by developers not part of the pcb-rnd team. External plugins are not hosted in the pcb-rnd svn and are not supported by the pcb-rnd team.
A core plugin can be disabled (not compiled at all), static linked (builtin), or dynamically linked (plugin). This is selected during pcb-rnd ./configure time. An external plugin is always dynamically linked.