What are Extension Controller's?

Hi,

A custom controller is an Apex class that implements all of the logic for a page without leveraging a standard controller. Use custom controllers when you want your Visualforce page to run entirely in system mode, which does not enforce the permissions and field-level security of the current user.

A controller extension is an Apex class that extends the functionality of a standard or custom controller. They are used when you

• to leverage the built-in functionality of a standard controller but override one or more actions, such as edit, view, save, or delete.
• to add new actions.
• to build a Visualforce page that respects user permissions.

For more you can refer to the standard apex guide.

Hope this helps you.

You want extend existing controller's functionality, leverage the native controller's actions or Custom controller's enhanced functionality.

Controller Extensions are also useful as a way to organize functionality, providing more modularity and readability.

<apex:page controller="Account" extensions="AccountControllerExtension[,extension2,extension...]">

thanks