I am a newbie. I want to know how Ruby is different from Java. When to use Ruby and what are the advantages? Is it focused entirely on Web?
Visit http://www.ruby-lang,org. There’s lots of info there on how it compares to other languages.
It’s a dynamic language with many uses. Ruby on Rails, a framework written in the Ruby language, is designed for the web. Ruby itself can be used for many things, including web, automation, shell scripting, and more.
Ruby is to Java as Rails is to Spring+Hibernate (I’m oversimplifying here to get the point across.)
There’s this site called google.com and if you typed your subject in the query textfield all of your questions would have been answered 
http://www.google.com/search?client=safari&rls=en&q=ruby+vs+java&ie=UTF-8&oe=UTF-8
Best regards
Peter De Berdt
Get your hands dirty and try for yourself. Its the best way to find out. There are several free books available on the web. Programming Ruby 1st edition is one of them.
They are just tools
Above all try to have fun using them
Jorge