Functional Testing occurs out to be the core of all testing domains. The primary objective that occurs is that the name itself indicates a quality assurance of the software through a functional point of view. What you see/view over the screen, you require to test it. It can be Java API or it could be a net web service. You require to validate what the interface occurs out to be supposed to provide you. Specifically, often, you would not be informed regarding the business requirements, & yet you occur to be expected to come up with a good tested software product.