Ruby On Rails is an open source web application framework that's designed to help you rapidly build apps in the Ruby programming language.
The system is based around the Model, View, Controller architecture. So Rails Models represent the data of an application and rules to manipulate it; Views are the user interface; and Controllers provide the logic that connects models and views together. This arrangement helps in keeping business logic isolated from GUI code, and clearly separating code into different types and places makes for easier maintenance, too.
Rails includes several tools to help you get started. With Scaffolding, for instance, entering a single command will quickly generate multiple files representing all the most important parts of your application: very helpful, especially if you're a beginner (or just in a hurry).
And if you need extra assistance then there are more than 1,500 plugins currently available, to extend your forms, improve security, add search features, internationalisation options and a whole lot more.
This download includes the Ruby language and RubyGems server, which you'll require to download Ruby on Rails itself. Check out the Rails Guides for help on installing Rails and creating your first application.