In the world of Windows Phone development, the cornerstone app architecture is called MVVM (Model-View-ViewModel). In this video, I talk through some of the structure of an MVVM application to talk about how we build MVVM apps for the phone to take advantage of benefits like data binding, commanding and ViewModel portability.
MVVM is an enormous topic. In addition to this video, you should take a look at the following articles, videos and blog posts to learn more about the why and the how of MVVM.
These two posts are targeted at Windows Phone Mango (7.5) but all the code carries across to Windows Phone 8 as well.
Laurent Bugnion, who leads the fantastic MVVM Light Toolkit project, has a 4-article MSDN series on MVVM
And finally, more videos: