How to Tell the Difference Between a Virus, a Worm and a Trojan
- 1
How do you diagnose whether your computer issue is a virus, a worm or a Trojan? These three forms of malware--software that hurts your computer--differ in the ways they infect you computer, what they can do once triggered and in how they spread. - 2
THE VIRUS: At heart, a virus is just a computer program. Within its repertoire of mischief, it can slow your computer's performance, cause crashes or display unwanted images and popup ads. A virus attaches itself to another program, usually an executable file--i.e. a file that has to be run (in contrast to a file that just contains data). The virus is not triggered until you run the executable file. Common file extension names for executable files include .bat, .com and .exe--so when opening an email attachment with these extensions make doubly sure that the sender is a trusted source. The good news is that because the virus is only triggered by running the file it's attached to, it takes the action of a human to further the spread of a computer virus. - 3
THE WORM: A worm is actually a particular type of virus. Unlike the standard virus, worms do not need human interaction to spread. They stand alone and do not need to attach themselves to an executable file. Instead, they can replicate themselves and then spread to other systems. Many worms are designed to tap into your saved email addresses and send themselves to everyone on your contact list. For this reason, it's a good idea to make the first contact on your list a fake, such as the name AAA AAAA at the email address AAA@AAA.org. That way, if a worm taps into your contacts and attempts to spread itself, you'll get a delivery error email from AAA AAAA, and you'll know to address the problem! Worms don't typically attack your computer directly, but by flooding your contact lists, and then your contact's contact lists, etc., they can very quickly consume all available bandwidth and create a network jamming mess! - 4
THE TROJAN: A Trojan horse, like the Greek legend from which it draws its name, is a program that poses as something harmless but becomes troublesome once you've downloaded it. Unlike a virus or a worm, the Trojan does not replicate itself or infect other systems. However, once on your computer the Trojan can create a "backdoor" to your computer that allows others to gain complete access to your system. Once they've entered through the backdoor, hackers can cause all kinds of spooky mischief, such as printing documents on your computer, opening your cd tray, or flipping your desktop image upside down. Worse, they can access your files and view confidential information.
Source...