This page talks about computer Viruses, how to detect them, how to prevent them, and has links to webpages with even more and better info than is listed here. A computer virus is a set of unwanted instructions that are executed on your computer resulting in a variety of effects.
Viruses can be rather docile, such as simply annoying the user with a sound.
These scripts often result in undesired instructions and effects. Good programmers do a thorough check of their work prior to releasing it to the public. Viruses are made for a variety of reasons. Some viruses have been created by disgruntled employees that want revenge on a company's product. Others are created to destructively celebrate certain holidays and raise the voice of certain religious and governmental fanatics.