ExtJS 4.x was based on MVC (Model-View-Controller) architecture. Ext JS supports MVVM (Model-View-ViewModel) architecture since Ext JS 5.
In the MVVM architecture, the job of Model and View remains the same as MVC.
Model is a collection of data fields which persist through the data package. They can be linked to other models through associations and linked to a data stream through the use of a proxy.
View is any type of components that output information on to your browser, i.e. Container, Form, Grid, Chart.
ViewModel is a class that manages data for the specific view.
ViewController: Ext JS 5 introduced ViewController which will be attached to a specific view and will handle all the events of that view.
You will learn about Ext JS MVVM folder structure in the Ext JS Folder Structure section. Visit Sencha documentation for the detailed information on the application architecture.