What Is The Difference Between an IoC Container and MEF?
MEF allows you to write your application as a series of much simpler mini applications, and have those brought together to create a much great whole
IoC container = service level components, MEF = Autonomous Components. A MEF "part" will probably use an IoC container
I recommend reading them highly, they will do a much better job than I could at explaining the detail of the decision points, Should I use MEF for my general IoC needs? and Should I use MEF with an IoC container?