What I mean by this is if I install jRuby on Rails, can I use only the ruby language to build my application? I wanted to install my rails application with jRuby in case I wanted to use Java in the future, though I don't need it now. Or do I not understand? Is jRuby ruby that has access to java libraries?
yes, jruby is ruby (your ruby scripts need no modification to run under jruby). in addition, it has full access to java libraries - check out the wiki for how easy it is to integrate a java class or library into your ruby code.