In computing, a denial-of-service attack is a cyber-attack where the perpetrator makes a machine or network resource unavailable to its users. It temporarily or indefinitely disrupting services of a host connected to the Internet. Denial of service is typically accomplished by flooding the targeted machine or resource with superfluous requests in an attempt to overload systems and prevent some or all legitimate requests from being fulfilled. A DoS attack is analogous to a group of people crowding the entry door or gate to a shop or business, and not letting legitimate parties enter into the shop or business, disrupting normal operations.
Criminal perpetrators of DoS attacks often target sites or services hosted on high-profile web servers such as banks or credit card payment gateways. Revenge, blackmail andactivism can motivate these attacks. DDOS Attack.
Denial-of-service attacks are an explicit attempt by attackers to prevent legitimate users of a service from using that service. In a distributed denial-of-service (DDoS) attack, the incoming traffic flooding the victim originates from many different sources. This effectively makes it impossible to stop the attack simply by blocking a single IP address; plus, it is very difficult to distinguish legitimate user traffic from attack traffic when spread across so many points of origin. There are two general forms of DoS attacks: those that crash services and those that flood services. Also, many attacks involve forging of IP sender addresses (IP address spoofing) so that the location of the attacking machines cannot easily be identified and so that the attack cannot be easily defeated using ingress filtering. DDOS Attack.
In addition, Khan C. Smith testimony in Court shows us the first demonstration of DoS attack made by him in 1997. It happened during a Defcon event disrupting Internet access to the Las Vegas Strip for over an hour. Also, the release of sample code during the event led to the online attack of Sprint, Earthlink, eTrade, and other major corporations in the year to follow. DDOS Attack.
A distributed denial-of-service (DDoS) is a cyber-attack where the perpetrator uses more than one unique IP address, often thousands of them. The scale of DDoS attacks has continued to rise over recent years, by 2016 exceeding a terabit per second. 
Advanced persistent DoS
An advanced persistent DoS (APDoS) is to be perpetrated by an advanced persistent threat (APT). Actors who are well resourced, exceptionally skilled and have access to substantial commercial grade computer resources and capacity. APDoS attacks represent a clear and emerging threat needing specialised monitoring and incident response services and the defensive capabilities of specialised DDoS mitigation service providers.
This type of attack involves massive network layer DDoS attacks through application layer (HTTP) floods SQLi and XSS attacks. Typically, the perpetrators can simultaneously use 5 attack vectors involving up to several tens of millions of requests per second. Also, often accompanied by large SYN floods that can attack the victim and also any service provider or any managed DDoS. These attacks can persist for several weeks- the longest continuous period noted so far lasted 38 days. This APDoS attack involved approximately 50+ petabits (50,000+ terabits) of malicious traffic.
To conclude, Attackers in this scenario switch between several targets to create a diversion to evade defensive DDoS countermeasures. All the while eventually concentrating the main thrust of the attack onto a single victim. In this scenario, threat actors with continuous access to several very powerful network resources. Network Resources are capable of sustaining a prolonged campaign generating enormous levels of un-amplified DDoS traffic.