A Trojan (or Trojan horse) is a malicious
program contained inside apparently harmless programming. It can get control
and do its chosen form of damage, such as ruining your hard disk or allowing
someone to take over your computer remotely. In one case, a Trojan was in a
program that was supposed to find and destroy computer viruses. A Trojan may be found by antivirus software, but not always.
A Trojan can also be put onto our computer if we go to a malicious Web site. When the site is loaded, a series of pop-under pages can be loaded by another Web host, and one of those pop-unders can download a file to our computer. Once it gets on our computer, this particular Trojan redirects search engines such as google.com and altavista.com to a different search site. If your computer is infected, you may not even realize it. One clue is when a whole bunch of pornography and gambling popups start appearing. Trojans can pose a privacy risk, because its can get a list of what pages we have visited. All the better to 'target' the ads to us!
A program that neither replicates nor copies itself, but causes damage or compromises the security of the computer. Typically, an individual emails a Trojan Horse to you-it does not email itself-and it may arrive in the form of a joke program or software of some sort.