Hi Madhukar, Standard controllers offer the same familiar functionality you’ve seen and used before on standard Salesforce pages, such as save, edit, delete, and cancel. Associating your Visualforce page with a standard controller lets you create interactive components that call these actions without any additional code.
https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_controller_def.htm
https://developer.salesforce.com/page/Introduction_to_Controllers_and_Extensions
Have you googled?
http://salesforce.stackexchange.com/questions/4041/difference-between-controller-and-extensions
Standard controllers offer the same familiar functionality you’ve seen and used before on standard Salesforce pages, such as save, edit, delete, and cancel. Associating your Visualforce page with a standard controller lets you create interactive components that call these actions without any additional code.