Over the past months, there have been widely reported widespread attacks on computer systems perpetrated through malicious software, more commonly known as viruses, worms, and trojans. What are these? How do they differ from one another? Here are some definitions on what these treacherous programs are.

Viruses - Programs that modify other programs on a computer, inserting copies of themselves. Viruses are not distinct programs - they cannot run on their own, and need to have some host program, in which they are hidden, be executed to activate them.

Worms - Programs that propagate from computer to computer on a network, without necessarily modifying other programs on the target machines. Worms can run independently and travel from machine to machine across network connections; worms may have portions of themselves running on many different machines. Worms do not change other programs, but they may carry a payload, another program that does (for example, a true virus).

Trojan horses - Programs that appear to have one function but actually perform another function. Trojan horses are named after the Trojan horse of myth which the Greek army used to capture the city of Troy. Analogous to their namesake, modern-day Trojan horses resemble a program that the user wishes to run - a game, a screen saver, or an editor. While the program appears to be doing what the user wants, without the user’s knowledge, it also does something else unrelated to its advertised purpose.

Bacteria, or rabbits - These programs make copies of themselves to overwhelm a computer system’s resources. Bacteria do not explicitly damage any files. Their sole purpose is to replicate themselves. A typical bacteria program may do nothing more than execute two copies of itself simultaneously on multiprogramming systems, or perhaps create two new files, each of which is a copy of the original source file of the bacteria program. Both of these programs then may copy themselves twice, and so on. Bacteria reproduce exponentially, eventually taking up all the computer's processor capacity, memory, or disk space, denying the user access to those resources.

Logic bombs - Programmed code within a program that lie dormant for an extended period of time until they are triggered; at this point, they perform a function that is not the intended function of the program in which they are contained. They are often triggered when the computer's internal clock arrives at a certain date and time or when a certain event occurs.

Droppers - This program is not a virus, nor is it infected with a virus, but when run, it installs a virus into memory, on to the disk, or into a file. Droppers have been written often as a carrier for a virus, and sometimes as an act of sabotage. Some anti-virus programs try to detect droppers.