You can see the original benscomputer.no-ip.org version of this post in the archive
So someone has mentioned the term 'Computer Virus' and you're wondering what it means? Put simply a Computer Virus is a program that attaches itself to other programs on your system without your consent.
To look at it in a bit more depth requires a greater understanding of Malware as a whole, when the average user refers to a computer virus they may actually be referring to worms, Trojan Horses, rootkits, spyware or an actual virus. You may also hear users refer to them by the incorrect plural time virii (Virii is actually an incorrect plural Latin term for men) what they actually mean is Viruses. Knowing both terms can however be beneficial when trying to find information on a virus. However you refer to any of the above, they all come under the category Malware, which is simply a term for software that does something unwanted.
Spyware is generally referred to separately from Viruses etc, because it's behaviour is generally completely different, although some Trojan Horses and Viruses will act as spyware.
Most malware can be avoided by running an Anti-Virus (AV) program, but it is important to keep the virus definitions up to date. There are numerous available online with prices ranging from free to 400+.
Now that the lines of distinction are somewhat blurred, lets take a look at each of these types of malware.
A true computer virus acts much like that of a biological virus, replicating itself with no regard for the impact upon its host (in this case your computer) adding it's own little bit of code to any executable files it finds, this can be a very slow process or an almost instantaneous process. Some will perform a very specific action (The Vampire subclass of Virus will gradually remove characters from your screen) whilst some exist simply to exist. Many viruses are written by hobbyists, mainly as a proof of concept, or to show that they can. The true aim of a virus for most authors is to write a bit of code that can hide itself from process designed specifically to look for malware, it's a challenging if somewhat unethical area of programming.
When a virus (or any other malware for that matter) is released, it is known as being 'in the wild.' Many viruses that are written are never let into the wild, they are simply written as a challenge for the programmer, tested and then removed. Of course, there are those that are unethical enough to let their code loose onto the Internet. On occassion a piece of code will not behave as expected and may escape into the wild without the programmer ever planning to release it.
As mentioned before, many viruses are written as a proof of concept. For example the first Linux virus seen in the wild - Bliss - simply attached itself to executable files to prove that it could, and even included an uninstaller!
Viruses tend not to propogate between different computer systems very effectively, and those that do are often referred to as worms.
Sadly, there are those with ill intent that write viruses which is probably how you have come to hear the term.
Trojan Horses are programs that pretend to be something they are not, and then perform their designed action. This is a common way of downloading a virus onto a users computer, and also leads to Spyware infections, Keyloggers etc. Generally a program will advertise free porn, Free Gamecodes or cheats but once run by the user will instead open backdoors (an access point into your system for all and sundry to log onto your computer for). Of more concern is the recent trial of a suspected pedophile where it was found that malware had created a child pornography server on the suspects computer without his knowledge. Is it just a very good defence, or is it true? Either way the very concept of that happening is an unthinkable development in the way that malware affects us.
The best way to avoid trojan horses is to scan every program before you run it, and avoid programs that promise to give you something for free.
Worms are essentially self propagating viruses, they behave in the same sort of manner but with added functionality. Many will email themselves to everyone in your contacts, inviting the receipient to look at a photo or watch a movie. An example of this would be the Iloveyou virus which offered users a naked picture of Anna Kournikova.
These can largely be avoided by being very, very careful which attachments you open, confirm with the sender that they actually sent it and scan it with you AV before opening.
For those interested, an Anti-Virus scanner detects viruses by looking for the signature left by a virus on an executable program. Because the virus adds itself to a program, this signature will always be there, the signature being a construct of code common to the virus. However viruses can be programmed to adapt, or be manually adapted by the author which is why you hear of Strain A of xxx virus, it is simply the same virus written to have a slightly different behaviour and/or signature.
This article was a very brief look at viruses and also touched upon other forms of malware, hopefully it has been of benefit to you. Although a dark world, the world of Viruses is very interesting, so consider doing some additional research into exactly what computer viruses do, and the motivation behind the writing of said code. The more you know about viruses as a whole, the easier you will find it to detect when your computer has contracted the electronic equivalent of Hepatitis and the more confident you will be in disinfecting it.