JRuby on Rails vs. Ruby on Rails

JRuby is the ruby implementation that runs on a JVM where as Matz’s ruby is a C implementation.

Key features to note are:

-Jruby having very good performing JVM and much faster than Ruby 1.8.6 (.i.e MRI) 
-It allows us to access some very popular Java libraries from Ruby
-It supports both Ruby 1.8.6 and Ruby 1.9 language specifications 
-It has very good support for Rails today
-It has very good support for Google App Engine today 
-It can use a multitude of web server configurations both Java and non-Java 
-It has support for true multi-threading because the Global Interpreter Lock (GIL) doesn't exist
-Jruby runs on Java VM's and it's either compiled or interpreted down to Java byte code.
-JRuby can integrate with Java code. If you have java class libraries (.jar's), you can reference and use them from within ruby code with JRuby. In the other direction you can also call Jruby code from within Java. JRuby can also use the JVM and application server capabilities.
-JRuby is usually hosted within Java application servers such as Sun's glassfish or even the Tomcat web server.
-Although you cannot use native ruby gems with JRuby there are jruby implementations for most of the popular ruby libraries