Context : OnPlatforms
MarcAndreesen distinguishes three types or levels : http://blog.pmarca.com/2007/09/the-three-kinds.html
First he notes that he's only talking about computer platforms meaning things that can be "programmed" in some way (presumably not, therefore, hardware platforms that can play different kinds of video format etc.)
level 1 : Access API : your application lives and executes outside the platform and makes calls to it via a public API)
level 2 : Plug-in API : Andreesen puts this in terms of it showing up inside the platform's UI : as in FaceBook. It helps you present your app. I might suggest that the more generic difference is that it involves a callback to your application from the platform. That makes, say, Windows a level 2 platform for Windows apps.
level 3 : Online Platforms : these host and execute the application code.
Andreesen pushes his Ning as a level 3 (excellent news, as Ning was always cool.)