I would rather use a different explanation than Marco:
A plug-in project is what you add to the IDE. Your real plugins live here. Eclipse organizes projects, and projects is a plugin for Liferay. Because the IDE interprets different projects differently (for example, stand-alone Java applications and Dynamic Web Projects, etc.), the Portlet Project or Thematic Project is simply added to this list.
When you have a plugin project (such as a Portlet), you can add as many portlets to it as you want. Thus, if you are developing portlets, you will always have at least one portlet. This portlet plugin can contain any number of portlets.
You will always deploy the entire plugin in Liferay - this fact may affect which portlets you want to group into one plugin.
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