Difference Between a Worm and a Trojan

Trojans and worms are nothing you want on your computer.

Worms and Trojans are two kinds of malicious software, or "malware," that get into your computer without your knowledge or consent, usually for a harmful purpose. The key difference is in how they spread.

Function

A worm is a program that makes copies of itself and distributes them to other computers connected by a network. A Trojan is a program that appears useful and legitimate but carries out harmful activities once installed.

Propagation

Worms can spread without any action by a computer user; for example, once a worm is in your system, it can automatically send copies to every name in your address book. A Trojan gets into your system because you were tricked into putting it there, such as by downloading a seemingly useful browser toolbar with the Trojan hidden in the code.

Effects

Worms and Trojans can have the same effects. They may do something immediately harmful, such as generating pop-up ads or logging keystrokes, or they may open "back doors" for someone else to take control of your computer.

"Trojan"

"Trojan" is a reference to the Trojan horse of "The Iliad." Greek soldiers hid inside a wooden horse presented to the city of Troy as a gift, then attacked once the horse was brought inside the city.

"Worm"

The origin of the name "worm" is less clear; because computer data was once stored on tapes, according to Slate magazine, it may be a play on tapeworms, parasites that live in the digestive tract.

References

• Microsoft: What is a Computer Worm
• Slate: What's Worse, a Virus or a Worm?