Pay for the service is the motto of cloud computing. It allows users to use distributed resources in the Internet to do their computations without installing and paying for those resources. Those resources are diverse to provide services that cover software, platform and infrastructure. When developing services, security is critical especially at cloud computing. Among the numerous cloud attacks that can target the cloud computing systems, Denial of Service (DoS) and Distributed Denial of Service (DDoS) attacks can cause a major problem in cloud security. One of the main objectives of DoS and DDOS is to consume large amount of server resources so that the server cannot provide service to normal users. Attackers usually gain access to large number of computers by exploiting the vulnerabilities to set up attack armies. This paper will present a comprehensive study for DoS attacks, different detecting techniques of DDoS will be analyzed and reviewed against different parameters, and present an experimental study showing the impact of DoS attacks in cloud applications. This research will measure the stability and validity of the questionnaire’s content using Cronbach's alpha, and will determine the impact of the related variables on the result using stepwise multiple linear regression analysis and spearman correlation.