This article is from the Computer Virus for New Users FAQ, by Nick firstname.lastname@example.org with numerous contributions by others.
A type of program that is often confused with viruses is a 'Trojan horse'
program. This is not a virus, but simply a program (often harmful) that
pretends to be something else.
For example, you might download what you think is a new game; but when you
run it, it deletes files on your hard drive. Or the third time you start
the game, the program E-mails your saved passwords to another person.
Note: simply downloading a file to your computer won't activate a virus or
Trojan horse; you have to execute the code in the file to trigger it. This
could mean running a program file, or opening a Word/Excel document in a
program (such as Word or Excel) that can execute any macros in the document.