A Denial-of-Service attack aims to exhaust the resources at the disposal of a server, network resources, disk, RAM, or a server's processors and can be part of a structured hack, for example to keep a router from giving a response to a peer to hijack a network session.
Currently there are three main variations:
Applicative DDoS and network DDoS use different methods (and require different defences), but usually the term DDoS refers to a network DDoS. With traffic volumes that become more and more massive (for those concerning the network), the traffic of the attacks reaches several hundred Gigabits per second. These attacks can touch any service: email, SSH, Web, you name it, but the web is mostly targeted.
Typically the 3 key parameters present in a Denial-of-Service attack are: