A Distributed Denial of Service (DDoS) attack is an attempt to make an online service unavailable by overwhelming it with traffic from multiple sources. They target a wide variety of important resources, from banks to news websites, and present a major challenge to making sure people can publish and access important information.
Attackers 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.
Specialized online marketplaces exist to buy and sell botnets or individual DDoS attacks. Using these underground markets, anyone can pay a nominal fee to silence websites they disagree with or disrupt an organization’s online operations. A week-long DDoS attack, capable of taking a small organization offline can cost as little as $150.
source – here