Ruby on Rails, often simply called Rails, is a powerful web application framework written in the Ruby programming language. It follows the convention over configuration principle, which allows developers to create applications quickly and efficiently by reducing the number of decisions they need to make. Rails is designed to make programming web applications easier by making assumptions about what every developer needs to get started. It emphasizes the use of MVC (Model-View-Controller) architecture, which helps in organizing application code and separating concerns, ultimately leading to cleaner and more maintainable code.