What do computer Viruses, Trojans, and Other Malware Actually Do?

A Trojan horse, or Trojan, in computing is a generally non-self-replicating type of malwareprogram containing malicious code that, when executed, carries out actions determined by the nature of the Trojan, typically causing loss or theft of data, and possible system harm. The term is derived from the story of the wooden horse used to trick defenders of Troy into taking concealed warriors into their city in ancient Anatolia, because computer Trojans often employ a form of social engineering, presenting themselves as routine, useful, or interesting in order to persuade victims to install them on their computers.

A Trojan often acts as a backdoor, contacting a controller which can then have unauthorized access to the affected computer.While Trojans and backdoors are not easily detectable by themselves, computers may appear to run slower due to heavy processor or network usage. Malicious programs are classified as Trojans if they do not attempt to inject themselves into other files (computer virus) or otherwise propagate themselves (worm).A computer may host a Trojan via a malicious program that a user is duped into executing (often an e-mail attachment disguised to be unsuspicious, e.g., a routine form to be filled in), or by drive-by download.

Malware, short for malicious software, is any software used to disrupt computer operation, gather sensitive information, or gain access to private computer systems. Malware is defined by its malicious intent, acting against the requirements of the computer user, and does not include software that causes unintentional harm due to some deficiency. The term badware is sometimes used, and applied to both true (malicious) malware and unintentionally harmful software.

Malware may be stealthy, intended to steal information or spy on computer users for an extended period without their knowledge, as for example Regin, or it may be designed to cause harm, often as sabotage (e.g., Stuxnet), or to extort payment (CryptoLocker). 'Malware' is an umbrella term used to refer to a variety of forms of hostile or intrusive software,including computer viruses, worms, trojan horses, ransomware, spyware, adware, scareware, and other malicious programs. It can take the form of executable code, scripts, active content, and other software.Malware is often disguised as, or embedded in, non-malicious files. As of2011[update] the majority of active malware threats were worms or trojans rather than viruses.

In law, malware is sometimes known as a computer contaminant, as in the legal codes of several U.S. states.

Spyware or other malware is sometimes found embedded in programs supplied officially by companies, e.g., downloadable from websites, that appear useful or attractive, but may have, for example, additional hidden tracking functionality that gathers marketing statistics. An example of such software, which was described as illegitimate, is the Sony rootkit, a Trojan embedded into CDs sold by Sony, which silently installed and concealed itself on purchasers' computers with the intention of preventing illicit copying; it also reported on users' listening habits, and unintentionally created vulnerabilities that were exploited by unrelated malware.

They corrupt data in the computer and open ports hence increasing vunerabilities and reducing system security and integrity

Viruses, trojans and malware can each perform different tasks dictated by their designated function.

Microsoft's Example of a Computer Virus is as follows

1. "A computer virus might corrupt or delete data on your computer, use your email program to spread itself to other computers, or even erase everything on your hard disk. Computer viruses are often spread by attachments in email messages or instant messaging messages."  

Kasperky's Definition of a Trojan is

1. "A backdoor Trojan gives malicious users remote control over the infected computer. They enable the author to do anything they wish on the infected computer – including sending, receiving, launching, and deleting files, displaying data, and rebooting the computer."

And UCS (University of Cambridge) gives the following definition for Malware

1. "Malware is a combination of the words 'malicious' and 'software' and is a piece of software (computer program) written by someone with mischievous or, more usually, malicious and/or criminal intent. The term malware includes, amongst other things, worms, trojan horses and spyware, as well as viruses."

So as you can see; malware is by definition a container of Viruses and Trojans. Each separate piece of software is designed for a specific need or purpose.

Virus : copies itself and infects computer files without users's permission or awareness(infect files)

Worm:spreads to other computers via network and are often designed to spread rather than actually harm (disrupt  network by consuming bandwidth) 

Trojan:appears harmless until executed , doesn't insert code into other files (taking control of the system it has infected, email sending , data destruction, logging keystrokes )

it will corrupt the files and folders. 

Cause major problems....which ultimately causes down time

Viruses

A computer virus is a type of malware that propagates by inserting a copy of itself into and becoming part of another program. It spreads from one computer to another, leaving infections as it travels. Viruses can range in severity from causing mildly annoying effects to damaging data or software and causing denial-of-service (DoS) conditions. Almost all viruses are attached to an executable file, which means the virus may exist on a system but will not be active or able to spread until a user runs or opens the malicious host file or program. When the host code is executed, the viral code is executed as well. Normally, the host program keeps functioning after it is infected by the virus. However, some viruses overwrite other programs with copies of themselves, which destroys the host program altogether. Viruses spread when the software or document they are attached to is transferred from one computer to another using the network, a disk, file sharing, or infected e-mail attachments.

Trojans

A Trojan is another type of malware named after the wooden horse the Greeks used to infiltrate Troy. It is a harmful piece of software that looks legitimate. Users are typically tricked into loading and executing it on their systems. After it is activated, it can achieve any number of attacks on the host, from irritating the user (popping up windows or changing desktops) to damaging the host (deleting files, stealing data, or activating and spreading other malware, such as viruses). Trojans are also known to create back doors to give malicious users access to the system.

Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate. Trojans must spread through user interaction such as opening an e-mail attachment or downloading and running a file from the Internet.

other Malware like worms are standalone software and do not require a host program or human help to propagate

Basm Allah Alrahman Alrahim

Virus destroy your data , hang your CPU ,and may open backdoors for Trojans .. .etc

Worms like virus but they don't need host program to spread

Trojans are the most dangerous , it will steal your credit card , password , secret information .. .etc

for more informations malware-part-1 in english malware-part-1 in arabic

Thank you for this question that help us to search and learn

its simple they slow,corrupt and destroy your computer

Spyware or other malware is sometimes found embedded in programs supplied officially by companies, e.g., downloadable from websites, that appear useful or attractive, but may have, for example, additional hidden tracking functionality that gathers marketing statistics. An example of such software, which was described as illegitimate, is the Sony rootkit, a Trojan embedded into CDs sold by Sony, which silently installed and concealed itself on purchasers' 

Trojan horses do not have the ability to replicate themselves like viruses; however, they can lead to viruses being installed on a machine since they allow the computer to be controlled by the Trojan creator.

A computer virus might corrupt or delete data on your computer, use your email program to spread itself to other computers, or even erase everything on your hard disk. Computer viruses are often spread by attachments in email messages/malware softwares or instant messaging messages.

It can also corrupt your system dlls in system32 and lead your computer to collapse.