What is a Virus?
In the computer world a virus is basically a self replicating program that someone has written to cause mischief. Some virus are relatively harmless and do things like randomly relocating your icons every time you start your computer. While others are malicious and can destroy your entire computer. Like an organic virus in the real world, there are several types. Some of the more common types are Worms, Boot sector, Macro, and Web scripting.
Worms spread using your email. They commonly read your address book and send themselves to everyone on it.
Boot sector hide in your hard drive in the first part that the computer reads when it is turned on. They can be very difficult to remove.
Macro most commonly occur in documents such as word and excel.
Web scripting hide in web sites waiting for you to visit with an unprotected computer.
There is also the well known Trojan Horse which in itself is not a virus. The purpose of the Trojan horse is to sneak into your computer and then release or let in a virus.
Just because nothing is going wrong with your computer does not mean you are not infected. Some virus purely collect information about you and your computer and then send it on to someone else waiting for your details. Some infected computers act as carriers that send the virus out to infect other machines. The last thing the writer of these types of virus want is for you to know it is there.
So now you have the basics about virus. Ensure you always have the latest version and updated antivirus on your computer.