This is a tutorial on how to build a Twilio application using the Visualforce Sites development environment inside Salesforce. The process involves creating an Apex class, components, and pages, which are then used to create a site that interacts with Twilio's APIs. The tutorial covers various steps such as setting up the Visualforce environment, creating the necessary controller, components, and pages, and wiring the application to a Twilio phone number. Once completed, users can build custom applications using Twilio within Salesforce, including features like call control, voice interfaces, and automatic reminder calls.