DDoS attack consists in "overloading" the attacked server or communication channels, and the exhaustion of the resources of the attacked system either degrades the quality of the application or stops its operation altogether. Simple DoS is rare and is usually fought off by banal blocking of access to the system for the attacking computer.

DDOS is a distributed attack based on the same principle; it is carried out from more than one attacking computer. DDoS complicates blocking attackers somewhat - the larger the attacking botnet, the more difficult it is to fight back.Network resources like web servers have a limit on the number of requests they can serve concurrently. In addition to the allowable load on the server, there are also restrictions on the channel's bandwidth that connects the server to the Internet. When the number of requests exceeds the performance of any component in the infrastructure, the following can occur:A significant slowdown in the response time to requests.
Denial of service for all or part of user requests.

As a rule, the ultimate goal of an attacker is to completely stop the operation of a web resource - "denial of service." The attacker can also demand money to stop the attack. In some cases, a DDoS attack may attempt to discredit or destroy a competitor's business.

DDoS attacks stand out in cyber risk landscape as they do not rely on malicious downloads or network access. These cyber attacks are designed to disrupt normal traffic on a target network or server.
This goal is usually achieved by an influx in traffic. Most commonly, bots or 'zombie computer' work together to target different layers within a company's network connection. Websites or other resources are left offline.
DDoS attacks don't usually occur in a single incident. They can happen over a long period of time, causing permanent damage and cost that will only get worse if it isn't addressed.

More info @ What is Managed DDoS Service