Visualforce is a framework that allows developers to build sophisticated, custom user interfaces that can be hosted natively on the Lightning platform.
Over the past several years, Salesforce has created a comprehensive platform for building on-demand applications. Like other sophisticated application development platforms, the Lightning platform offers separate tools for defining:
The structure of the data—that is, the data model
The rules that detail how that data can be manipulated—that is, the business logic
The layouts that specify how that data should be displayed—that is, the user interface