The program which has been purposely created to cause the computer problems, generally minor ones as a trick, but rarely very nasty ones certainly, such as erasing your total hard disk. Viruses were initially intended to attach themselves to programs on a disk, and then "hide" in the computer's memory once the host program is executed, and "infect" every disk they come across. Some types of virus which is famous like "I love you" send with the email, also an attachment, never open the attachment because it contents virus which will harm your computer.