Dependency Properties vs. Attached Properties

Attached Properties use an accessor style that XAML parser can use, i.e. SetPropertyName
As opposed to regular dependency properties that use regular Set/Get accessors.

For attached properties see: http://msdn.microsoft.com/en-us/library/cc265152(v=vs.95).aspx

For regular dependency property requirements see Checklist for Defining a Dependency Property
Nice sample of custom dependency object here (bottom)