Backdoor - A security hole that is designed to allow someone to connect to your computer and control it. Can be installed by a legitimate program's author, or by malware.

Firewall - A system that analyzes network traffic and blocks unwanted traffic from entering or leaving a network. Used at the border of a network to prevent people on the internet from accessing internal network services.

Intrusion Detection System - A computer system that monitors network traffic for data that is known to be part of an attack, or activity that is forbidden by policy.

Keystroke logger - A type of spyware which records the list of keys that are pressed and sends them to an attacker. Often used to discover passwords.

Malware - any program which intentionally causes harm to the computer.

Network Security Scanner - A program used to scan computers over a network to determine the types of attack that the computer may be vulnerable to.

Spyware - A program that records personal information about the user and reports it to a 3rd party. Often used to track browsing habits and deliver ads. The user is usually tricked into installing the spyware. Many times these programs are poorly written and tend to crash or slow down the user's computer.

Trojan - A program which appears to be good, but is intended to cause harm.

Virus - a computer program which copies itself into other programs so that when the user runs the infected program the virus gets another chance to spread.

Worm - A program which spreads through a network without human intervention. Often relies on security vulnerabilities or misconfigurations to propagate.