Malware

Malware is software designed to infiltrate or damage a computer system without the owner's informed consent. It is a portmanteau of the words "malicious" and "software". 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.

Many normal computer users are however still unfamiliar with the term, and most never use it. Instead, "(computer) virus" is used in common parlance and often in the general media to describe all kinds of malware. Another term that has been recently coined for malware is badware, perhaps due to the anti-malware initiative Stopbadware or corruption of the term "malware".

Software is considered malware based on the perceived intent of the creator rather than any particular features. It includes computer viruses, worms, trojan horses, spyware, dishonest adware, and other malicious and unwanted software. In law, malware is sometimes known as a computer contaminant, for instance in the legal codes of California, West Virginia, and several other U.S. states.

Malware should not be confused with defective software, that is, software which has a legitimate purpose but contains harmful bugs.

Purposes
Many early infectious programs, including the Internet Worm and a number of MS-DOS viruses, were written as experiments or pranks generally intended to be harmless or merely annoying rather than to cause serious damage. Young programmers learning about viruses and the techniques used to write them might write one to prove that they can do it, or to see how far it could spread. As late as 1999, widespread viruses such as the Melissa virus appear to have been written chiefly as pranks.

A slightly more hostile intent can be found in programs designed to vandalize or cause data loss. Many DOS viruses, and the Windows ExploreZip worm, were designed to destroy files on a hard disk, or to corrupt the filesystem by writing junk data. Network-borne worms such as the 2001 Code Red worm or the Ramen worm fall into the same category. Designed to vandalize web pages, these worms may seem like the online equivalent to graffiti tagging, with the author's alias or affinity group appearing everywhere the worm goes.

However, since the rise of widespread broadband Internet access, more malicious software has been designed for a profit motive. For instance, since 2003, the majority of widespread viruses and worms have been designed to take control of users' computers for black-market exploitation. Infected "zombie computers" are used to send email spam, to host contraband data such as child pornography, or to engage in distributed denial-of-service attacks as a form of extortion.

Another strictly for-profit category of malware has emerged in spyware -- programs designed to monitor users' web browsing, display unsolicited advertisements, or redirect affiliate marketing revenues to the spyware creator. Spyware programs do not spread like viruses; they are generally installed by exploiting security holes or are packaged with user-installed software.


http://en.wikipedia.org/wiki/Malware