DDoS, or Distributed Denial of Service. In Greek it translates as "Distributed Attack on Denial of Service".
A DDoS attack targets a system, usually a server or an entire data center. The attack "bombs" the system with a huge amount of data from different sources. And when the system, in its attempt to respond, ends up running out of resources, it overloads and eventually collapses.
Example: a few minutes after New Year, Easter, we all have difficulty making a call or sending an SMS.
This is because too many subscribers are trying to use the network at the same time!
In DDoS attacks the scale is clearly larger, as they rely on up to hundreds of thousands of devices. Quite large DDoS attacks and can drop entire data centers with multiple servers.
Major companies such as CNN, Amazon, Yahoo, eBay, Sony, Facebook, Twitter, and even the FBI site, among others, have fallen victim to such attacks.
Historically, the largest DDoS attack ever recorded took place on October 21, 2016.
The victim was the company Dyn, which controls most of the DNS (Domain Name System) infrastructure of the Internet. This case has affected countless websites worldwide, which relied on Dyn's DNS servers.
The amount of data they managed, with the help of the Mirai botnet, to bring the servers to their knees probably exceeded 1.2Tbps (Terabit per second, trillions of bits per second).
Many times, a group of hackers can create a botnet and then rent its use to third parties, whom we call "well-meaning".