A Visualforce controller is a set of instructions that specify what happens when a user interacts with the components specified in associated Visualforce markup, such as when a user clicks a button or link. Controllers also provide access to the data that should be displayed in a page, and can modify component behavior.
The Force.com platform provides a number of standard controllers that contain the same functionality and logic that are used for standard Salesforce pages. 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.
A standard controller exists for every Salesforce object that can be queried using the Force.com API.