Ruby on Rails is a web development framework written in the Ruby programming language. It is designed to make programming web applications easier by making assumptions about what every developer needs to get started. It allows you to write less code while accomplishing more than many other frameworks and languages.
Rails is a model–view–controller (MVC) framework, providing default structures for a database, a web service, and web pages. It encourages and facilitates the use of web standards such as JSON or XML for data transfer, and HTML, CSS and JavaScript for display and user interfacing. In addition to MVC, Rails emphasizes the use of software engineering patterns and principles, such as Convention over Configuration (CoC), Don't Repeat Yourself (DRY), and the Active Record pattern.
There are plenty of resources available to help learn Ruby on Rails. These include books, websites, screencasts, and IRC channels.