Definition:
"A parasitic program written intentionally to enter a computer without the users permission or knowledge. The word parasite is used because a virus attaches to files or boot sectors and replicates itself, thus continuing to spread. Though some virus's do little but replicate others can cause serious damage or effect program and system performance. A virus should never be assumed harmless and left on a system." -Symantec
A functional definition of a computer virus must include the fact that it creates damage to the computer system "infected"; either inadvertently or deliberately. This definition is somewhat simplified and does not cover all virus types, but is sufficient to show the major differences between viruses and so-called "Trojan" programs, which is that the virus replicates, but the Trojan program does not.
Many of us are computer users. (I bet you're using one right now!) Whether we are systems analysts or just home users, the fact that we have access to computer technology gives us a good reason to become aware of computer viruses and the precautions to take to keep from being infected.
Prevention is the key to success when dealing with viruses. The following are links that were designed to help you obtain a better understanding of viruses and their prevention:
The Administrative Computing Services of the Buffalo State University
of New York has a very nice definition:
http://wings.buffalo.edu/computing/adc/tips/fprot/virus.html
WPI also has a great definition with some extra information on
various types of viruses:
http://tardis.res.wpi.edu/~virus/definition.html