Hi ramakrishna,
That's a really good question. but first of all we need to know exactly what do you want to integrate with you existing web application?
jBPM5 exposes the APIs for building your UIs in any technology that you want so you need to pick the right set of APIs (the process engine APIs or the Human Task APIs) and build your screens using them. Usually most of the people wants to integrate the tasks lists inside your existing apps and for that you will only need to comunicate with the Human Task component.
Cheers