Denial of Service refers to an attack in which a malicious actor attempts to force a service to fail, usually by overloading it. A Denial of Service (DoS) takes advantage of a design failure or vulnerability in a software. For example, sending a particular packet might make a service unanvailable.
Something as simple as turning off the power could be seen as a denial of service. If you have limited bandwidth and overload your network by downloading huge gigabytes of files and slow down everyone else, you’re also essentially performing a denial of service.
DDoS stands for Distributed Denail of Service. This is when multiple devices attempt to send traffic to one central point to overload their resources. This is generally performed by computers that have been turned into botnets.
If you are the victim of a DDos attack, you might be able to filter out traffic patterns at the firewall. You may have to contact your internet service provider to see if they have anti-DDoS systems and help to stop the attack before it reaches you. Third party hardware and software solution also exist to help mitigate DDoS attacks.