Viruses, Trojans and Worms
Imagine this. You're sitting at home on your computer, doing your taxes or filling out an online credit card application containing highly personal information. Meanwhile, someone on the other side of the world is copying down everything you're typing and planning to use that information in malicious ways. Frightening? Absolutely! Impossible? No way.
Every day several new types of viruses and worms appear on the internet. These types of threats are generally described as "malware" and they can do anything from steal personal information on computers to launch massive internet attacks against certain web sites.
Viruses were likely one of the first types of dangers that you heard about. Decades ago, you had to worry about infected files on floppy disks and the damage viruses could cause on your PC. A virus is a piece of software that inserts itself into documents on your computer whether we're talking about something you've written, an audio file or even a movie. When you open a virus-laden file your computer creates a copy of the virus and it spreads throughout your machine, much like the flu virus spreads within a person. Viruses can cause various types of effects ranging from destruction of files, erasing of your entire hard disk or damage to programs on your computer.
A worm, on the other hand, is a program that actually runs on your computer and can even spread itself. It doesn't have to hide within a file. Worms can cause major damage to your machine. A worm could potentially launch an attack on other computers out on the internet, find sensitive files on your machine and steal information from them or even log all of the keystrokes you type during the day. Scary? For sure!
Trojans are essentially a type of worm. However, a Trojan is something that a user must reach out and install himself. While a worm can spread itself automatically between machines a trojan relies on a person, you, for example, to download and run its payload. Trojans often rely on tricking users into thinking that a file is safe enough to run, sort of like the story of how the Greeks built a giant wooden horse filled with soldiers and enticed the Trojans to let the 'horse' in their city. Trojans and worms typically have the same effect on your computer, it's just matter of how the malware gets in.
To make matters more interesting, some of the really scary malware is actually designed to be silent on your computer, watching what you do, recording personal information and possibly sharing your secrets with criminals around the world. Definitely not good.