A distributed denial of service (DDoS) attack is when an attacker, or attackers, attempt to make it impossible for a service to be delivered. This can be achieved by thwarting access to virtually anything: servers, devices, services, networks, applications, and even specific transactions within applications. In a DoS attack, it’s one system that is sending the malicious data or requests; a DDoS attack comes from multiple systems.
Generally, these attacks work by drowning a system with requests for data. This could be sending a web server so many requests to serve a page that it crashes under the demand, or it could be a database being hit with a high volume of queries. The result is available internet bandwidth, CPU and RAM capacity becomes overwhelmed.