Denial of service attack was a stretch of Distributed Denial of Service Attack (DDoS). DDoS comes the largest security risks and problems facing Internet users. This risk affecting obstruct with work of any system of targeted organizations, which leads to the harassment of system customer. This attack succeeds by exploiting several weaknesses to access resources in the target organizations. The attacker exhausts all capacity of resources in a period short time. The previous method leads to the denial of using any organization's resources from any authorized user. This paper illustrated DDoS attack and flooding concept have been proposed. This paper deals in general with describing structural approach of DoS attack in different levels of services. In addition, content explains the motivations of the attackers to use different attacks. In particular, the concept of DDoS attack has been clarified. Also describing types of different flooding attack with examines SYN-flood and flooding attack.
