Trojan horse is a program that appears harmless or even helpful, but after being executed performs an undesirable and malicious action. For instance, a Trojan horse can be a program advertised to be a patch, harmless application such as a calculator or a product upgrade or enhancement. The trick is to fool the user to download and install the program. Once executed, the Trojan horse can perform the function it was actually designed to do. This might include crashing a system, stealing data, and corrupting data.
Worms are similar to viruses in that they replicate, but they do not require a host file to spread from system to system. The difference between viruses and worms is that a worm does not attach itself to an executable program as do viruses: A worm is self-contained and does not need to be part of another program to propagate itself. This makes a worm capable of replicating at incredible speeds. This can cause significant network slowdowns as the worm spreads.
A worm can do any number of malicious actions, including deleting files and sending documents via email without the user knowing. A worm can also carry another program designed to open a backdoor in the system used by spam senders to send junk mail and notices to a computer. Once this backdoor access is open to the computer, your system, it is vulnerable and open to data theft, modification, or worse.
Spyware is a new threat that can be very hidden and easy to get. Spyware is designed to monitor activity on a computer, such as Web surfing activity, and send that information to a remote source. It is commonly installed along with a free program that might have been downloaded.
Spyware detection software is becoming increasingly popular and given the information that can be stolen, should be considered an important part of a secure system.
One final consideration is that of virus hoaxes. The threat of virus activity is very real, and, as such, we are alerted to it. Some take advantage of this to create elaborate virus hoaxes. Hoaxes will often pop up on the computer screen or arrive in the email warning of a virus or claiming that your system has contracted a virus. These are more annoying than dangerous but serve to confuse and complicate the virus issue.