Standard controller :
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 Controller object that is automatically provided for standard and all custom objects, bindable to a Visualforce page component with the “standard Controller” attribute. It provides a reference to a single/list of record to a set of common actions for data processing and default navigation.
What are the tasks of standard controller?
Standard controllers provides ability to access and interact with structured business data contained in records displays in the proper user interface.
Standard controller tasks: Controlling data, Controlling actions and Controlling navigation.