A Denial-of-Service attack (DoS) is when someone arranges for a lot of computers to open a website. This stops everyone else from accessing the website as there is too much internet traffic flowing to it. People with slower internet connections, such as dial-up, are affected more by attacks.
Types
There are different kinds of DoS attacks:
Flood attack - This is when a system gets too much internet traffic (people trying to connect to it). The traffic uses bandwidth and the internet servers slow down and eventually stop.
Logic and software attacks - Internet packets are sent that are supposed to use bugs in the software or system. These attacks are easier to defend against because firewall or software patches usually correct the problem.
Distributed Denial-of-Service (DDoS) attack - This type of attack uses either flood attacks or logic attacks, but it uses many different computers under the attacker's control (see Botnet). This type of attack is one of the most often used, and usually against company websites. This type of attack is often the hardest to prevent, track, and stop.
History
In the 1990s and early 2000s, many web companies were hit by DoS attacks. For example, Microsoft was affected by DoS attacks in early 2001. Many users were unable to access the Microsoft website and many Microsoft services. On June 10, 2014, Evernote was targeted by a DDoS attack for 24hours. [source?]