What are computer viruses, worms, and Trojan horses?

What is a virus?

A computer virus, according to Webster's Collegiate Dictionary, is "a computer program usually hidden within another seemingly innocuous program that produces copies of itself and inserts them into other programs or files and that usually performs a malicious action (as destroying data)". Two categories of viruses, macro viruses and worms, are common today. Computer viruses are never "naturally occurring"; they are always man-made. Once created and released, however, their spread is not directly under human control.

Macro viruses

A macro is a piece of code that can be embedded in a data file. Some word processors (e.g., Microsoft Word) and spreadsheet programs (e.g., Microsoft Excel) allow you to attach macros to the documents they create. In this way documents can control and customize the behavior of the programs that created them, or even extend the capabilities of the program. For example, a macro attached to a Microsoft Word document might be executed every time the document is saved and cause the text of the document to be run through an external spell checking program.

A macro virus is a virus that exists as a macro that is attached to a data file. In most respects, macro viruses are like all other viruses. The main difference is that they are attached to data files rather than executable programs. By some estimates, 75% of all computer viruses now in existence are macro viruses.

Worms

Worms are very similar to viruses in that they are computer programs that replicate themselves and often, but not always, contain some functionality that will interfere with the normal use of a computer or a program. The difference is that unlike viruses, worms exist as separate entities; they do not attach themselves to other files or programs. Because of their similarity to viruses, worms are often also referred to as viruses.

What is a Trojan horse?

A Trojan horse is a program that does something undocumented which the programmer intended, but that the user would not approve of if he or she knew about it. According to some people, a virus is a particular case of a Trojan horse, namely one which is able to spread to other programs (i.e., it turns them into Trojans too). According to others, a virus that does not do any deliberate damage (other than merely replicating) is not a Trojan. Finally, despite the definitions, many people use the term "Trojan" to refer only to a non-replicating malicious program.