Ruby on Rails (often called Rails) is a powerful, open-source web application framework written in Ruby. It follows the Model-View-Controller (MVC) architecture and emphasizes convention over configuration (CoC) and don't repeat yourself (DRY) principles. Rails simplifies web development by providing built-in tools and libraries for tasks like database management, routing, and session handling, allowing developers to quickly build and deploy scalable, maintainable web applications. It's widely used for building dynamic, data-driven websites and is known for its developer-friendly environment and productivity-focused design.