MalwareMalware, or malicious software, is programming which is designed to take advantage of any weaknesses in your computer's security setup. It has been purposely designed by an ethically-challenged coder to secretly install itself on your machine and to bring some benefit only to him or herself - certainly not to you. It may take the form of spyware or adware, or a worm, trojan, keylogger, rootkit or any of a number of dubious pieces of code. In the following video, technology analyst Colton Travers explains in more depth what malware is, why you don't want it on your computer and what you can do about it.
Wikipedia has a detailed if rather technical description of malware in its various forms at en.wikipedia.org/wiki/Malware.