Computer Worms vs. Trojan Horses vs. Scareware vs Spyware

Computer worms use the network to send copies of themselves to other PCs, usually exploiting a security hole to travel from one host to the next without user intervention. Because they can spread so rapidly across a network infecting every PC in their path, they tend to be the most well-known type of malware, although many users still mistakenly refer to them as viruses.

Trojan horses are applications that look like they are doing something innocuous, but secretly have malicious code that does something else. In many cases, trojans will create a backdoor that allows your PC to be remotely controlled, either directly or as part of a botnet—a network of computers also infected with a trojan or other malicious software. The major difference between a virus and a trojan is that trojans don't replicate themselves—they must be installed by an unwitting user.

Scareware is a relatively new type of attack, where a user is tricked into downloading what appears to be an antivirus application, which then proceeds to tell you that your PC is infected with hundreds of viruses that can only be cleaned if you pay for a full license. Of course, these scareware applications are nothing more than malware that hold your PC hostage until you pay the ransom—in most cases, you can't uninstall them or even use the PC.

Spyware is any software installed on your PC that collects your information without your knowledge, and sends that information back to the creator so they can use it in some nefarious way. This could include keylogging to learn your passwords, watching your searching habits, changing out your browser home and search pages, adding obnoxious browser toolbars, or just stealing your passwords and credit card numbers.