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. |
Thys Michels Blog 