A denial-of-service (DoS) or distributed denial-of-service (DDoS) attack is an attempt to make a machine or network resource unavailable to its intended users. A DoS attack generally consists of efforts to temporarily or indefinitely interrupt or suspend services of a host connected to the internet.

Distributed denial-of-service attacks are sent by two or more people, or bots, and denial-of-service attacks are sent by one person or system. As of 2014, the frequency of recognized DDoS attacks had reached an average rate of 28 per hour. A week long DDoS attack campaign may be purchased online for $150

Attackers frequently build networks of infected computers, known as 'botnets', by spreading malicious software through emails, websites and social media. Once infected, these machines can be controlled remotely, without their owners' knowledge, and used like an army to launch an attack against any target. Some botnets are millions of machines strong.

Botnets can generate huge floods of traffic to overwhelm a target. These floods can be generated in multiple ways, such as sending more connection requests than a server can handle, or having computers send the victim huge amounts of random data to use up the target’s bandwidth. Some attacks are so big they can max out a country's international cable capacity.

DDos attacks are usually characterized by the following:

Unusually slow network performance
Unavailability of a particular web site
Inability to access any web site
Dramatic increase in the number of spam emails received
Disconnection of a wireless or wired internet connection
Long term denial of access to the web or any internet services

Denial of service attacks can be problematic, especially when they cause large websites to be unavailable during high-traffic times. Fortunately, security software has been developed to detect DoS attacks and limit their effectiveness.

http://afritechnet..ae/2015/06/what-is-ddos.html