MVVM

Models are simple class objects that hold data. They should only contain properties and property validation. They are not responsible for getting data, saving data, click events, complex calculations, business rules, or any of that stuff.

Views are the UI used to display data. In most cases, they can be DataTemplates which is simply a template that tells the application how to display a class. It is OK to put code behind your view IF that code is related to the View only, such as setting focus or running animations.

ViewModels are where the magic happens. This is where the majority of your code-behind goes: data access, click events, complex calculations, business rules validation, etc. They are typically built to reflect a View. For example, if a View contains a ListBox of objects, a Selected object, and a Save button, the ViewModel will have an ObservableCollection ObectList, Model SelectedObject, and ICommand SaveCommand.