Visualforce and the Native User Interface
Force.com provides many places for Visualforce pages to be integrated into its native user interface. You can embed Visualforce pages inside standard user interface pages, override the buttons that navigate between pages, override the standard pages entirely, and add buttons and tabs to navigate to an entirely custom user interface. Areas of the native user interface extensible through Visualforce are summarized here:
Standard pages—Standard pages provide the default user interface for maintaining records in the Force.com database. These pages can be overridden with your custom Visualforce pages.