Rails is an open-source framework that provides developers with a set of tools and libraries for building web applications. Created by David Heinemeier Hansson, Rails was designed to emphasize software engineering principles that prioritize simplicity, readability, and productivity.

• Convention over Configuration (CoC) : Rails follows the principle of "convention over configuration," meaning that it makes assumptions about the best way to do things, reducing the need for explicit configuration and boilerplate code.
• Don't Repeat Yourself (DRY) : Rails encourages the DRY principle by minimizing repetition in code, promoting modular design, and providing built-in helpers and generators.
• Active Record : Rails includes the Active Record ORM (Object-Relational Mapping) framework, which simplifies database interactions by mapping database tables to Ruby objects and providing a rich set of methods for querying and manipulating data.
• RESTful Architecture : Rails promotes RESTful design principles, making it easy to build RESTful APIs and web services.

To start building applications with Rails, you'll need to install Ruby and Rails on your system. Once installed, you can create a new Rails application using the Rails new command and follow the conventions and best practices of the framework to develop your application.

1. Productivity : Rails' emphasis on convention over configuration and the DRY principle allows developers to write less code and focus on solving business problems, leading to faster development cycles.
2. Community and Ecosystem : Rails has a vibrant community and a rich ecosystem of gems (libraries) that extend its functionality, making it easier to integrate with other technologies and services.
3. Scalability : Rails applications are scalable and can handle a large number of users and transactions, thanks to its efficient caching mechanisms, database optimizations, and support for distributed computing.

Ruby on Rails is a powerful and versatile framework that offers developers a productive and enjoyable experience for building modern web applications. Whether you're a seasoned developer or just starting your journey in web development, Rails provides a solid foundation for creating robust, maintainable, and scalable applications.