Malware

Malware is a generic term used to describe virus, trojan horses, worms and other types of programs which are use to attack computers on the Internet, both the machines onto which they are (usually) surreptitiously placed *and* the ones which they in turn attack, either by promulgating themselves further, or in some other way.

Not all malware is surreptitious, some comes along with "free" software unexpectedly, either by defaulting a "should I also install" checkbox to yes (we're looking at you, Oracle/Java), or by simply installing the ancillary program you didn't expect.