-- jBPM Capabilities and what it can do ?
>> Probably best to start with the community Docs: http://docs.jboss.org/jbpm/v5.3/userguide/
-- System Behaviour (What it does) ?
>> Chapter 1 probably answer your question here: http://docs.jboss.org/jbpm/v5.3/userguide/ch.overview.html#d0e26. Chapter 2: http://docs.jboss.org/jbpm/v5.3/userguide/ch.getting-stated.html has some good info as well.
-- Live examples of jBPM
>> For this is probably best to search this user forum, and also follow the jBPM Blog : http://kverlaen.blogspot.com/ and the Drools/jBPM blog: http://blog.athico.com/. Very often you will find articles there with community applications or prototypes that use jBPM and Drools
-- How it will be helpful for us as a workflow engine ? Whether it meets our needs or not ?
>> Typically the anwer here is "yes" ;) But get familiar with what jBPM does first and make up your own opinion.
-- Is it used for business or application development ?
>> jBPM is a Business Process Management Suite and it a great bridge between business users and application developers.
-- What are its components ?
>> I believe chapter 1.2 in the docs will help you with this question: http://docs.jboss.org/jbpm/v5.3/userguide/ch.overview.html#d0e64. You can also google several of the jBPM Presentations by the code jBPM Developers to find very good slides of the modularity of jBPM components.
Hope this helps.