With the increase in development of internet in past few years, online attacks are also incremented. DDoS attacks are dangerous threats among them. DDoS attacks are the process of making the target system non-responsive to the legitimate requests. They were focusing on network and transport layers initially. Now the application layer DDoS attacks are prominent and are most difficult to resolve online. This paper presents a comprehensive study on Botnet-based DDoS attacks on application layer, and the extended incidents of such attacks that are clearly increased recently.