… a "platform" is a system that can be reprogrammed and therefore customized by outside developers
users -- and in that way, adapted to countless needs and niches that the platform's original developers could not have possibly contemplated, much less had time to accommodate.

In contrast, an "application" is a system that cannot be reprogrammed by outside developers. It is a closed environment that does whatever its original developers intended it to do, and nothing more.

— Marc Andreessen, "Analyzing the Facebook Platform, three weeks in" (http://blog.pmarca.com/2007/06/analyzing_the_f.html)