Extending versus customizing an ext JS widget/component with the web UI framework

Extending and customizing are very similar. Both involve changes to the default, out-of-the-box version of the application.

Extending is a type of customization that involves creating changes in a separate file that are applied to the application, making these changes easy to identify and easy to remove.

Customization can also involve more direct changes to the application that change (and do not preserve) the original configuration of the application.