Visualforce Standard vs Custom Apex Chart

The Visualforce chart below is key to understand how customization and development work in Salesforce.

Standard Behavior Custom Behavior
Standard Look and Feel Application Framework and Default UI (Page Layout)Example: Standard Opportunity page with a default page layout. Page Layouts and Custom Apex ClassesExample: Standard Opportunity page that validates the opportunity stage.
Custom Look and Feel Visualforce pages with standard controllersExample: Visualforce page containing opportunity information Visualforce pages with custom Apex ControllersExample:Opportunity search portal to search for specific opportunities in Salesforce.