If you are new to the concepts of UI Test Extensions I would suggest the following references as I am not going to go over these concepts in depth as they have already been covered in more detail than I can offer. What I can do is point you in the direction of the material I used to help myself understand the Concepts.
This blog post by Venkatesh Sargunesan from Visual Studio Team Test is a great in depth overview of how the UITest Architecture is laid our and functions under the covers. Understanding how it currently works will help you to understand the Extensibility points.
This is the first and at the moment the only video I have found that walks you through the concept of Extending the UI Test Framework. It is really just a preview; I would have liked it to walk you through the complete creation even if it was just for a simpler project. This would demonstrates how the individual cells in Excel cannot be accessed by the UI Test Framework, then after installing an extensions rich property information of each cell can be retrieved.
This is where you will begin to really understand the concepts of UITest Extensions. It will walk you through creating a basic sample extension that writes to the UITest log and then walks you through how the Excel sample in the previous link works.