- Spyware. Any software that covertly gathers user information through the user's Internet connection without his or her knowledge. Once installed, the spyware monitors user activity and transmits that information in the background to someone else. Spyware can also gather information about e-mail addresses, passwords and credit card numbers.
- Trojan horse. A Trojan horse is a malicious program that is disguised as legitimate software. When a Trojan is activated on your computer, the results can vary. There are several different types of Trojans: Remote Access Trojans (RATs), Backdoor Trojans (backdoors), IRC Trojans (IRCbots), and Keyloggers. Many of these different types can be employed in one Trojan. Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate.
- Virus. A virus is a self-replicating program that attaches itself to a program or file so it can spread from one computer to another, leaving infections as it travels. Some viruses cause only mildly annoying effects while others can damage your hardware, software, or files. Almost all viruses are attached to an executable file, which means the virus may exist on your computer but it cannot infect your computer unless you run or open the malicious program.
- Worm. A worm is a special type of viruses that can replicate itself, but a worm is self-contained and does not need to be part of another program to propagate itself. They are often designed to exploit the file transmission capabilities found on many computers. A worm uses a network to send copies of itself to other systems. Due to the copying nature of a worm and its ability to travel across networks the end result in most cases is that the worm consumes too much system memory, or/and network bandwidth, causing Web servers, network servers, and individual computers to stop responding. Some worms can tunnel into your system and allow malicious users to control your computer remotely.
- Zombie. A zombie computer is a computer attached to the Internet that has been compromised by a cracker, a computer virus, or a trojan horse. Generally a compromised machine is only one of many in a "botnet", and will be used to perform malicious tasks of one sort or another, under remote direction. Most owners of zombie computers would be unaware that their system was being used in this way. Zombies have been used extensively to send e-mail spam; between 50% to 80% of all spam worldwide is now sent by zombie computers.
- Other Malware.
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