There are many varied reasons why individuals or organisations carry out these attacks and also many different methods. For example, DDoS attackers may target their competitor’s websites, especially if it is an e-commerce site, they may also wish to disrupt news, financial organisations such as banks, and even DNS servers.
One way of thinking about how these attacks take place is to imagine a Call Centre, selling products, which has (for arguments sake) 50 incoming telephone lines. If 50 people called and got through, then all of the incoming lines would be busy, this would mean that nobody else with a genuine sales enquiry would be able to get through to buy something. In simple terms this is how a DDoS attack works, but instead of telephone lines, it hogs most or all of the available Internet bandwidth available to the computer or resource. In a nutshell it is all about limiting or disabling communication.
The good news is that there are several ways of stopping such attacks from taking place. However, varying levels of protection means varying costs, meaning that the most robust form of protection can cost a small fortune.
In the UK, denial-of-service-attacks are a criminal offence and can lead to a maximum prison sentence of up to 10 years.