The following types of malicious software and activity are briefly defined:

• Virus
• Worm
• Trojan
• Spyware
• Keylogger
• Adware
• Rootkit
• Ransomware
• Phishing
• Vishing
• Pharming
• Bot attack
• Greyware
• SPAM

Virus

A computer virus is designed to copy itself and propagate from one computer file to another, usually by attaching itself to program files. When the infected program is run it may infect other files. Human intervention is necessary for a virus to spread between computers and systems. This may be accomplished by e.g. downloading files, exchange of diskettes/CDs/DVDs, copying files to and from file servers, and sending infected email attachments.

Worm

A worm will infect other computers. A network worm will also spread automatically in a network and is independent on human interaction. Thus, worms are faster spreaders than viruses. Normally worms will neither delete nor change files. However, they may be loaded into computer memory, and thereby result in slower computer performance.

Trojan

A trojan is a program that seems to be genuine and even useful, and thereby tricks the users to install/use it. When the program is run however, it will perform other (often additional) actions, like setting up backdoors so that hackers may access your computer.

Spyware

Spyware is programs that collect information about a person or an organization without that entity's consent and awareness. Users are often installing spyware without being aware of it; either by downloading something presumed to be useful, or by selecting/clicking something in a pop-up window.

Keylogger

Keyloggers are normally used as spyware. They are programs, which log the keys pressed on a keyboard and send this to a third party through a communication channel. Keyloggers may be used e.g. for stealing passwords.

Adware

Awware is software that enables displaying banner advertisements when the program is running. Adware may register your Internet behaviour, and is often mentioned in connection with Spyware, because both may register and report user information to a third party.

Rootkit

Rootkits are software used to hide files, running processes, Registry entries, or other kinds of data. Although a rootkit may be used for legitimate purposes, it is a fact that this technology is often used by authors of malicious software to hide the malware they try to distribute.

Ransomware

This term is used for those instances when a malicious program encrypts files on a computer, and (usually) leaves a message that a certain ransom has to be paid for the decryption key to be disclosed. Ransomware are usually distributed as email attachments. The encryption process is executed when the attachment is opened.

Phishing

The word phishing is derived from fishing. It is used to decribe attacks where someone attempts to get your personal information, like passwords, credit card numbers, bank account numbers etc. Fake (spoofed) emails that without closer inspection seem to originate from legitimate organizations and corporations are typical tools for the phishers. These emails try to trick you into believing that you get a real request from your bank or a similar corporation, in order for you to release private information to the person with malicious intent. The phishers are often hard to track down, and may also operate from countries where the legislation system does not prohibit this kind of activity.

Vishing

This is a special kind of Phishing (see above) where Voice over IP (VoIP) is used as the communication channel instead of the more usual email technique. The term "vishing" is derived from VoIP and phishing.

Pharming

Pharming is a sophisticated, special form for phishing. The pharmers utilize the DNS system; i.e. the system on the Internet that translates a computer name to an Internet Protocol address (IP address).  The pharmers attempt to change the host name to another IP address than the real legitimate address. This enables the pharmer to e.g set up a web page that looks like a bank's web page, and thereby collect lots of personal information. Another (older) word for pharming is DNS poisoning.

Bot attack

Several high profile web pages (including Norman's) have been victims of so-called Distributed Denial of Service (DDoS) attacks These assaults have been carried out by a set of bot (robot) computers that simultaneously send lots of requests to a special computer or a certain network.  The amount of data is so huge that the computer(s) under attack are unable to handle the load, and are thereby rendered inaccessable for their intended, legitimate tasks.  The users of the computers that are parts of such an attack, are often unaware of this fact - thus the computers are often called zombie computers. The person that control the attack - "the brain behind" - is located elsewhere and just presses a key to initiate the attack.

Computers that are part of a network which itself is controlled by a robot is often called a botnet.

Greyware

The term greyware is used for software that may be viewed as useful in some instances, but which also includes component(s) that may be seen as malicious or annoying in other contexts.

SPAM

Spam is not malicious software in its pure sence. Spam is unsolicited information, and may perhaps be called "nuisanceware". The most common channel for SPAM is email, although any communication channel may be used (web, Instant messaging etc.). It is estimated that email-based SPAM constitutes far more than 50% of all email traffic through the Internet.

The spammers use different techniques to attempt to avoid the spam to be detected by antispam software, e.g. using images as all or parts of the text (image spam).