Distributed Denial of Service (DDoS) attacks have become a major threat to the Internet community because DDoS attacks are regularly launched by well organized and widely spread botnet computers that are concurrently and accordingly sending large amount of traffic or service request to the target system. The target system either responds so slowly or crashes completely. These attacks not only congest a Server, but also affect the performance of other Servers on the entire network also, which are connected to Backbone Link directly or indirectly. The focus of this study, based on existing literature, covers the architecture or models of DDoS attacks and DDoS attack tools, propose taxonomies to characterize the scope of DDoS attacks and categorize it based on their types, and also the recent trends of DDoS attacks on web server are studied and evaluated.