The term “virus” has become a generic way of describing what is otherwise known as malware. Malware, short for malicious software, is a category of software developed for malicious purposes. It is intended to cause damage, loss of access to information and programs, or theft. Calling malware a virus is often a misnomer. A virus is only one specific type of malware. They fall into several different categories, each having unique characteristics.
Viruses and worms are unique from other malware in that they are able to self-propagate. That is to say they have the ability to make copies of themselves. They do this to either attempt to infect other systems or to embed themselves more securely into an infected system. Both are capable of performing the same kinds of malicious tasks.
Much like an actual virus relies on the infection of host cells in a body, a computer virus relies on host software. A computer virus cannot run on its own. Instead, it injects itself into host application, usually by replacing core files with itself. When the infected application runs, the virus becomes active and executes its malicious code. When someone using an infected computer shares infected files with others, the virus spreads.
As a measure to disguise the existence and location of the virus, infected applications usually continue to function as expected. On occasion, some viruses may completely overwrite the host application, leaving only itself in its place. In many cases, there may be few obvious indications of infection.
Worms, unlike viruses, do not depend on host applications or people to spread and cause damage. A worm is a self-contained application capable of running independent of any other software. They can install and run all on their own. Worms can produce copies of themselves, seek out other systems, and transfer themselves to infect the other systems. They do so without any human interaction.
Ransomware is a class of malware that is described by how it impacts an infected system. Its purpose is to deny access to files, programs, or entire systems. Typical ransomware encrypts its target, making files, folders, or hard drives inaccessible. Most ransomware prompts the user with a demand for payment in exchange for the key to unlocking the encrypted files.
There are numerous variations of ransomware. Some are viruses, infecting application files and depending on human file sharing activities to spread. Other versions are worms that self-propagate and wreak havoc on entire networks without any human interaction.
The Trojan horse takes its name from Greek history. It is nothing but a container disguised as something desirable. Trojan horses are malicious software, but they do not have their own “payload”. They are instead used to deliver other malicious programs onto systems. Typically a virus or a worm.
Crafty malicious programmers develop Trojan horses to look like legitimate files or programs. They can appear to be software installers for desirable games or simply appear to be an music or movie. Trojan horses are most commonly spread through file-sharing sites and applications like Pirate Bay or Limewire. Users download a file, thinking they are getting a free copy of the latest version of Adobe Photoshop and inadvertently install malware onto their computer.
Spyware stands alone as a unique category of malware. The purpose of spyware is to record a user’s activities on a computer and report it to somewhere else. Spyware can be simple key-loggers that recording every keystroke, they can report on Internet browsing activities, or can even be programs that provide remote access to cameras and microphones attached to the infected computer.
While some versions of spyware are viruses and worms, many are not. There is an entire gray market (see shady) for spyware that anyone can purchase and intentionally install on their own computer. People do this to track activities of their spouses, children, or other’s who might share the same computer. It should not go without warning that the intentional use of spyware carries serious risk of legal issues due to potential violations of individual rights to privacy.