In this blog post Matthias Shapiro explains what a PART is and how to create one for your custom control in Silverlight 3.
There are basically five steps in creating a PART. I think you can actually do it in less, but I’m trying to follow what I think is proper coding practice. In this tutorial, we’re going create parts out of a Button and also a Storyboard (partly because there are different ways to assign a PART if it is in located in the control resources like a Storyboard is).