Virus - A program that can "infect" other programs by modifying them to include a, possibly evolved, copy of itself. A program that infects a computer by attaching itself to another program, and propagating itself when that program is executed. A computer can become infected by files downloaded over a network, or by the installation of new software or floppy disks that are infected with viruses. Some viruses are only pranks, and perform harmless actions like displaying a screen with a joke message on it. Others can destroy files or wipe out a hard drive. To avoid damage from viruses, write-protect the boot disk and other important disks, check new software or disks for viruses, and have virus protection software installed on the computer at all times. Disinfectant programs must be updated periodically because new viruses get into circulation over time. There are some virus protection programs available on the Internet for free. Knowingly spreading a computer virus is a crime punishable by law. (definition from TechDictionary.com)

Trojan horse - An apparently useful and innocent program containing additional hidden code which allows the unauthorized collection, exploitation, falsification, or destruction of data. A program or file that appears to be useful and harmless but which has harmful side effects such as destroying data or breaking security on the system on which it is run. It is similar to a virus except that it does not propagate itself as a virus does. (definition from TechDictionary.com)

Worm - A computer program that can make copies of itself, instead of spreading from file to file they spread from computer to computer, infecting an entire system using up resources in affected computers or causing other damage. They can penetrate the computer's memory from a computer network, calculate network addresses of other computers and send their own copies to these addresses. Such "viruses" (Worms are not viruses in the strict sense) sometimes start files on the system disks, but can't apply themselves to computer resources (with the exception of main memory) generally. (definition from TechDictionary.com)

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