Malware, short for malicious software, is software designed to secretly access a computer system without the owner's informed consent. The expression is a general term used by computer professionals to mean a variety of forms of hostile,intrusive, or annoying software or program code.

Software is considered to be malware based on the perceived intent of the creator rather than any particular features. Malware includes computer viruses, worms, trojan horses, spyware, dishonest
adware, scareware, crimeware, most rootkits, and other malicious and unwanted software or program.

In law, malware is sometimes known as a computer contaminant, for instance in the legal codes of several U.S. states, including California and West Virginia.

Computer virus

A computer virus is a computer program that can copy itself and infect a computer. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability. A true virus can spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive.

Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.

Computer worm

A computer worm is a self-replicating malware computer program. It uses a computer network to send copies of itself to other nodes (computers on the network) and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a virus, it does not need to attach itself to an existing program. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.

Trojan horse

A Trojan horse, or Trojan, is malware that appears to perform a desirable function for the user prior to run or install but instead facilitates unauthorized access of the user's computer system. "It is a harmful piece of software that looks legitimate. Users are typically tricked into loading and executing it on their systems", as Cisco describes. The term is derived from the Trojan Horse story in Greek mythology.

Spyware

Spyware is a type of malware that can be installed on computers and collects little bits of information at a time about users without their knowledge. The presence of spyware is typically hidden from the user, and can be difficult to detect. Typically, spyware is secretly installed on the user's personal computer.
Sometimes, however, spywares such as keyloggers are installed by the owner of a shared, corporate, or public computer on purpose in order to secretly monitor other users.

Adware

Adware, or advertising-supported software, is any software package which automatically plays, displays, or downloads advertisements to a computer. These advertisements can be in the form of a pop-up. The object of the Adware is to generate revenue for its author. Adware, by itself, is harmless; however, some adware may come with integrated spyware such as keyloggers and other privacy-invasive software.

Crimeware

Crimeware is a class of malware designed specifically to automate cybercrime. The term was coined by Peter Cassidy, Secretary General of the Anti-Phishing Working Group to distinguish it from other kinds of malevolent programs. Crimeware (as distinct from spyware, adware, and malware) is
designed (through social engineering or technical stealth) to perpetrate identity theft in order to access a computer user's online accounts at financial services companies and online retailers for the purpose of taking funds from those accounts or completing unauthorized transactions that enrich the thief controlling the crimeware.

Crimeware also often has the intent to export confidential or sensitive information from a network for financial exploitation. Crimeware represents a growing problem in network security as many malicious code threats seek to pilfer confidential information.

Rootkit

A rootkit is software that enables continued privileged access to a computer while actively hiding its presence from administrators by subverting standard operating system functionality or other applications.
The term rootkit is a concatenation of "root" (the traditional name of the privileged account on Unix operating systems) and the word "kit" (which refers to the software components that implement the tool). The term "rootkit" has negative connotations through its association with malware.