JRuby is a JavaTM implementation of the Ruby interpreter. While retaining many of the popular characteristics of Ruby, such as dynamic-typing, JRuby is integrated with the Java platform. With JRuby on Rails, you get the simplicity and productivity offered by Ruby and Rails and the power of the Java platform offered by JRuby, thereby giving you many benefits as a Rails developer, including the following:
Access the rich set of Java libraries from Rails applications
Use the powerful and secure support of Java Unicode strings with Rails applications
JRuby on Rails applications can spin off and pool multiple threads because JRuby uses Java threads, which map to native Ruby threads
To learn more about JRuby, see the JRuby Home page.