A virus is a program which propagates from one computer to the next by piggy-backing a ride in other files. After the virus has gotten a ride to it's new location, it infects the new host, and tries to arrange a ride to another host.
We won't discuss computer viruses in detail, other then to say that they occur mostly on personal computers. Although technically possible, viruses are very rare on computers running modern multi-user operating systems (such as Unix). The memory and file protection models provided by modern operating systems tend to inhibit the spread of viruses.
A common myth is that email viruses are spreading on the net. There are no email viruses that will ``infect'' your computer when you read them. The ``Good Times!'' email virus is a hoax.
The closest thing to an email virus is the new class of Microsoft Word and Excel viruses which have recently been discovered. These viruses are macros which are included in a word or spreadsheet file. You can protect yourself by turning off the automatic execution of macros in your default configuration for Microsoft products.