Ruby on Rails, often called RoR, or just Rails, is an open source web application framework written in Ruby that closely follows the Model-View-Controller (MVC) architecture.
It strives for simplicity and allowing real-world applications to be developed in less code than other frameworks and with a minimum of configuration.
The Ruby programming language allows for extensive metaprogramming, of which Rails makes much use. This results in a syntax that many of its users find to be very readable.
Rails is primarily distributed through RubyGems, which is the official packaging format and distribution channel for Ruby libraries and applications.
The Wikipedia entry for Ruby-on-Rails can be found here.
In the video below, tenthconcept demonstrates through a video how to use of AJAX in the Ruby on Rails web development framework.