Most known attacks against networked systems may be categorized into one or more of the following types of attacks. In many cases, attackers exploit a particular design and/or implementation vulnerability or weakness to gain unauthorized access or perform unauthorized actions. Knowledge of these attacks will help you defend against them more efficiently.
Denial of service is a kind of attack in which the victim (a networked system) is overwhelmed by requests for service from a single attacker, thus making it effectively unable to serve legitimate users (resulting in denial of service). The primary defense against denial of service attacks is network-level packet filtering and firewalls.