Viruses, Trojans and Worms. What's the Difference?

Viruses, Trojans and worms are all bad, right? True enough, but a little information can help you better protect yourself against these threats.

Virus: A computer virus is a program, a piece of software that attaches itself to other software or data. Like any software program, a virus must be run before it can accomplish its purpose. This is why one of the key acts of self-defense against viruses is to never click on an unfamiliar e-mail attachment. Clicking on the attachment, whose name and description may promise that it's something pleasant or worthwhile like a baby photo or a new program, actually starts the virus program. Once started, many viruses immediately mail copies of themselves to everyone in your e-mail program's address book or on your Instant Message friends list.

Trojan or Trojan horse: Named for the famous wooden horse of Greek history, Trojan horses lure unsuspecting users into downloading them by appearing to be something valuable and enticing, such as a game or photo viewer. Once a Trojan horse is "invited" into a computer, it prepares to wreak havoc, much like the Greek troops waiting inside the hollow horse to sack the city of Troy. The modern version may forward passwords and other sensitive information to the criminals who created it, corrupt data or delete files, or even create "backdoors" into your computer, allowing an outside network to take control of your computer and use it to mount large-scale attacks. Unlike viruses, Trojan horses don't make copies of themselves. They're often distributed by viruses, however, and some—called "droppers"—plant other viruses as part of their "payload."

Worm: Computer worms are software programs that copy themselves and, generally, immediately begin transmitting those copies over networks such as the Internet. Unlike viruses, which disguise themselves by lurking inside other programs, worms are self-contained. They are also exceptionally dangerous and powerful, capable of replicating throughout hundreds of thousands of computers in a matter of hours or even minutes. Large worm outbreaks have actually slowed the performance of the entire Internet. Worms are not only used for attacks, however; like Trojan horses, some worms are designed to burrow through computer networks seeking and creating "backdoors" that give computer control and access to criminals and hackers.

Follow-up: The most important thing to know about viruses, Trojan horses, and worms is that both new ones and improved versions of existing ones appear daily, which is why it's crucial to update your anti-virus software regularly.