MVC and MVVM
AngularJS is often described as an MVC ("Model-View-Controller") framework. Here is how this is often illustrated:
This picture, however, is far too simple.
First, only the most trivial applications can be understood as consisting of a single model, a single view and a single controller. More commonly, an application will include multiple views, multiple controllers, and multiple data models. So, it might look more like this:
The figure above makes another important substitution, however. "Controllers" are replaced with "view models". Angular can be better understood as a "MVVM" ("Model-View-ViewModel") framework. In this approach, we have "view models" mediating between views and (data) models. While this may seem like just a minor change of terminology, the idea of "view model" helps clarify the path towards better AngularJS architecture. A view model is a mediating object that takes data from a data model and presents it to a view in a "digested" form. Because of that, the view model superficially looks like a model. It should not be confused with the application's real data models. Misusing the view model as the model is one of the most common sources of problems in AngularJS.
Now let's see how MVVM model is realized in AngularJS.