Visualforce is the framework in Salesforce CRM that allows you to further customize your organization's user interface beyond the standard functionality that we have previously covered.
As described previously, using Visualforce, you can combine data from multiple objects, create mash-ups with data from external web services, and even override some of the logic and the behavior found within standard Salesforce CRM application functions. Visualforce consists of the following three elements:
Visualforce pages: These are used to define the user interface
Visualforce components: These can be thought of as a library of standard or custom-built sections of Visualforce code
Visualforce page controllers: These are used to control the behavior of Visualforce pages, and can either be controlled by standard logic, or you can create custom logic to change or extend the standard Salesforce CRM behavior