About Controllers and Extensions:

A standard controller:  consists of the same functionality and logic that is used for a standard
Salesforce page. For example, if you use the standard Accounts controller, clicking a Save button
in a Visualforce page results in the same behavior as clicking Save on a standard Account edit
page. If you use a standard controller on a page and the user doesn’t have access to the object, the
page will display an insufficient privileges error message. You can avoid this by checking the user’s
accessibility for an object and displaying components appropriately.

A standard list controller enables you to create Visualforce pages that can display or act on a set
of records. Examples of existing Salesforce pages that work with sets of records include list pages,
related lists, and mass action pages.

A custom controller is a class written in Apex that implements all of a page’s logic, without
leveraging a standard controller. If you use a custom controller, you can define new navigation
elements or behaviors, but you must also reimplement any functionality that was already provided
in a standard controller. Like other Apex classes, custom controllers execute entirely in system
mode, in which the object and field-level permissions of the current user are ignored. You can
specify whether a user can execute methods in a custom controller based on the user’s profile.

A controller extension is a class written in Apex that adds to or overrides behavior in a standard
or custom controller. Extensions allow you to leverage the functionality of another controller while
adding your own custom logic.