Visualforce is a component-based user interface (UI) framework that enables the creation of dynamic, reusable interfaces.Content Continues Below
Controllers consist of instructions that specify what happens when users interact with page components, such as when a user clicks a button. Page components can be controlled by the standard logic for Salesforce pages, or developers can associate their own logic with a controller class. Server-side standard controllers simplify basic database operations, such as queries and updates.
See an introduction to Visualforce: