A virus attaches itself to a program or computer file, and spreads from one computer to another, infecting each computer as it travels. Almost all viruses are attached to a legitimate, executable file or piece of software, which means it may exist on a computer but it can't infect until a malicious program is opened or run on the computer. It must have human action to spread, such as running an infected program, so a person can unknowingly continue its spread by sharing infected files or sending emails with viruses as attachments. They can be transmitted through emails or downloaded files and can be on CDs, DVDs, USB drives or any other digital media. The virus can be anything from just a prank that pops up messages to the complete destruction of programs and data.

Worm:

A worm is similar to a virus, and is considered a sub-class of a virus. Worms spread from one computer to another and do not require any human action. The worm can copy itself from one computer to another, and can send themselves to an entire email address book, and then everyone in those email address books, etc., making them capable of spreading to millions of computers within hours. Worms can hide in parts of operating systems that are invisible to a person, and it is common for them to only be noticed when they slow down the internet speed of infected computers, or halt computerized tasks. Some worms are created just to spread and not to be malicious while others carry a “payload”. A “payload” is a piece of code designed to do damage, such as deleting or encrypting files, or attaching itself to an outgoing email, sometimes allowing the creator of the worm to take control over the infected computers.

Trojan:

Trojan horses masquerade as a benign application but are actually malignant pieces of destructive software. Trojans do not spread or replicate but they instead introduce viruses into computer systems. The term originates from the Trojan War, when the Greeks gave a giant wooden horse to their foes (the Trojans), as a peace offering, but once inside the city walls the Greek soldiers came out of the horse's hollow belly and opened the city gates, allowing for the capture of Troy.

There are at least seven types of Trojan horses and they are all destructive:

• Remote Access Trojans (RATs) – these are designed to give an attacker complete control of a system. The attackers usually hide the Trojans in games or other small programs and the unsuspecting user then executes them on their devices.
• Data Sending Trojans – these are designed to give a hacker access to sensitive data such as passwords, credit card data, and email addresses, etc. This is done by the Trojan searching for specific pre-defined data, or by installing a keylogger that sends all recorded keystrokes back to the attacker.  
• Destructive Trojans – these are designed to destroy and delete files and acts more like a virus than a Trojan. These Trojans can often be undetected by anti-virus software.
• Proxy Trojans – these are designed to use the victim's computer as a proxy server, letting the attacker do anything from the computer as if they were the legitimate user, even using the victim's system to launch attacks against other networks.
• FTP Trojans – this Trojan horse opens the port for an FTP (file transfer protocol) and gives the attacker the ability to connect to the victim's computer directly through port 21 (the port for the FTP transfer).
• Security software disabler Trojans – these stop or kill computer security software or firewalls without the user's knowledge. This type of Trojan is usually combined with another type of Trojan as a “payload”.
• Denial-of-service (DoS) Trojans – the DoS Trojan is a type of attack that floods a network with useless traffic by exploiting limitations in the TCP/IP protocols. If the DoS attacks are known there are software fixes that system administrators can install to limit the damage, but new attacks are continually developed by the hackers.

Ransomware is a type of malware that prevents users from using their systems, either by locking the screen or their files; or it encrypts files, unless a ransom is paid. See [IDL:Beware of Ransomware.xml^Beware of Ransomware^Beware of Ransomware]. Ransomware can also be considered 'scareware' as it forces users to pay a ransom (fee) by scaring or intimidating them. For it to work, the computer needs to be infected with a virus, which is usually accomplished by tricking a user into clicking on an attachment or a link, and there is usually a sense of urgency in the message that suggests the user should hurry and click, such as winning a vacation or approving delivery of a parcel. Once the user clicks, the ransomware encrypts the hard drive, locking people out of their computer, and a screen appears threatening to destroy the files unless a ransom is paid.

Spyware:

This is used in malicious attacks such as phishing, social engineering and identity theft, and is designed to steal money from unknowing computer users. Spyware is often installed when a user downloads free software, but it cannot be spread such as a virus or worm. Spyware deceives a user by piggybacking on a piece of software, or by tricking the user into installing it, thinking they are installing a useful utility. Once installed, spyware programs collect personal information, such as internet searches, and can install additional software and re-direct web browser activity. Some spyware has keyloggers, which are small programs or hardware devices that monitor each and every keystroke a user types to record log-in names, passwords, or other sensitive information and send it on to hackers. Spyware can interfere with computer processes and halt applications by freezing them, cause difficulty when connecting to the internet, cause system-wide crashes or fail to allow computers to re-boot, change computer settings, change home pages, and cause loss of internet access or program functions.

Rootkit:

Rootkits bury themselves into an operating system (such as a 32 bit Windows operating system) and stealthily hide their files, processes and registry keys; or create hidden directories designed to keep them out of view from operating systems and security software. The attackers can then use the rootkit to hide their malicious software, such as spyware or keyloggers, and this will enable them to remotely access, record, modify, steal and transfer any information on a computer.

Adware:

Adware displays advertisements on a computer and redirects search requests to advertising websites, collecting data about the user, such as types of websites visited, so that the system will generate customized advertisements. Unauthorized Adware is malicious, such as the malware that uses Trojan-Spy behavior. Adware generally does not make its presence known, and it can run undetected on the computer, penetrating in the same manner as a virus. It can be included within some freeware or shareware programs or if a user visits an infected website this can result in unauthorized installation of Adware on the device.