What is 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 incorrectly used in common parlance and even in the media to describe all kinds of malware, though not all malware are viruses.

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

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

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