The documentation for
The Managed Extensibility Framework (MEF) describes its purpose as a tool that "...simplifies the creation of extensible applications". That is a very general statement. It's like describing nuclear physics as "the field of physics that studies the building blocks and interactions of atomic nuclei". In some ways MEF is as powerful to programming as nuclear physics is to science.
However, for the "common man", nuclear physics is only important because it makes your lights turn on when you flick the light switch. With MEF, we will use it to simply load a .xap file that contains a Silverlight application. Trust me, it can do a lot more, but MEF is in a state like the Visual State Manager was years ago, powerful but hard to use, so you may decide to wait for more advanced tooling to appear before implementing some of it's other functions.
You may have heard about MEF and how great it is. However, you may find it a bit confusing and hard to follow. Fear not. This article is designed for you! The goal here is to give you a quick easy win. You will be able to understand this one. You will come away with something you can use, and you will be able to tell your fellow colleagues, "MEF? yeah I'm using that, good stuff".
The full article is here: http://www.codeproject.com/KB/silverlight/MEFDynamicLoading.aspx