DDoS means Distributed Denial of Service.
A DDoS attack stops a service (online shop, online banking, online game). There are different motivations for a DDoS attack. It can be political reasons, unhappy customers, or it might have a criminal background. Blackmail for example.
Speaking about DDoS attacks, usually the first thing that pops up, is massive traffic, that overloads the Internet line. But there are 10 different ways of DDoS. 3 of them can only be stopped by your ISP. The remaining 7 types have to be blocked in your environment in front of your firewall.
To avoid these attacks, we use products from Corero.