“Malware, short for malicious software, consists of programming (code, scripts, active content, and other software) designed to disrupt or deny operation, gather information that leads to loss of privacy or exploitation, gain unauthorized access to system resources, and other abusive behaviour” http://en.wikipedia.org/wiki/Malware
“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, scare ware, crime ware, most rootkits, and other malicious and unwanted software or program.” http://en.wikipedia.org/wiki/Malware
Malware is usually poorly-programed which can cause your computer to become unbearably slow and very unstable. Apart from this they can also highjack your browser, redirect your search attempts, serve up nasty pop-up ads, track what website you visit and generally crash your computer.
In school we have software that checks what people are looking at which blocks pop-ups and other sites that are seem as security threats.