A custom controller is an Apex class that uses the default, no-argument constructor for the outer, top-level class. You cannot create a custom controller constructor that includes parameters. A custom controller is needed to perform complex validation that involves multiple objects and then redirect the user to another Visualforce page upon success. A custom controller can override the standard actions of a standard controller, such as save, edit, view, or delete, and define new actions. A custom controller runs in system mode, so the user's permissions and field-level security do not apply. References: You can find more information about custom controllers in the Visualforce Developer Guide and the Apex Developer Guide on Salesforce's official website.