Can anyone please provide me in layman's terms the difference between developing a JRuby and a Ruby, Rails application?
I use NetBeans as my Ruby on Rails IDE and every-time I create a project is asks me that question - and I don't really get the difference. Are there any pro and cons?
As far as I know jruby is simply a ruby implementation written in java so it allows easy integration with the java runtime.
JRuby is the Java implementation of Ruby. It's compatible with Ruby 1.8, and lags current Ruby (1.9). It runs on a JVM and allows you to easily integrate existing Java libraries.
Ruby runs within a native C based interpreter. JRuby runs inside the Java Virtual Machine. There are many gems and plugins that might not work inside JRuby, specifically one's with native C extensions, unless they've been rewritten to run in Java.
Ultimately the difference comes in when you want to actually deploy your application (assuming the gems you want to use weren't an issue). If you would like to deploy to a java web server, then JRuby would be the way to go. If you have a *nix environment, then Passenger + Nginx or Apache is very popular as well, which uses the standard ruby or ruby enterprise interpreter. There is always the mongrel or thin cluster option as well, again using the standard interpreter.
JRuby runs on the Java VM (the interpreter was written in Java), while the original Ruby interpreter was written in C. Both have up- and downsides also (Ruby can use native extensions, JRuby can access to Java types, objects etc.)
JRuby uses Ruby 1.8.6's syntax, 1.9.x will be available soon. JRuby has Java threads (meaning it will scale to many cpus, cores etc.), Ruby has some issues with userspace threading, locking ec.
Personally, I use JRuby with Glassfish v3 and Netbeans for RoR development, it scales a lot better than ruby + loads of mongrels, Apaches and it's easier to manage.
JRuby is mostly Ruby compatible. One aspect of JRuby is that you can run it in non-compatible modes for performance reasons.
Ruby is a programming language. JRuby is an implementation of the Ruby programming language.
Asking about the difference between the two doesn't really make sense: they are on two entirely different levels of abstraction.
It's similar to asking what is the difference between a car and a Prius. It doesn't make sense, either: a Prius an implementation of the general concept of a car. You could ask what's the difference between a Prius and a Hummer or the difference between a car and an aeroplane, but the difference between a car and a Prius doesn't make sense.
Similarly, with Ruby, you could ask about the difference between Ruby and Python or JRuby and MRI, but the difference between Ruby and JRuby doesn't make sense.