Any hacker tool intended to disable a user's personal firewall. Some will also disable resident anti-virus software

A program that overloads a connection by any mechanism, such as fast pinging, causing a DoS attack

A Distributed Denial of Service (DDoS) attack is one that pits many machines against a single victim. An example is the attacks of February 2000 against some of the biggest websites. Even though these websites have a theoretical bandwidth of a gigabit/second, distributing many agents throughout the Internet flooding them with traffic can bring them down. The Internet is defenseless against these attacks. The best defense is to run spyware software, and remove DDoS clients when they are found, so that their machines are not used as attack tools. Another approach is for ISPs to do "egress filtering": prevent packets from going outbound that do not originate from IP addresses assigned to the ISP. This cuts down on the problem of spoofed IP addresses