A virus is a small piece of software that does damage to your system. Older viruses used to distroy your data. Newer ones try to snoop on you when you are surfing and tyr to catch your bank or credit card login so they can steal your identity or your money. Sometimes the virus will use your computer to send spam emails.
They are very nasty. People make them because either they are criminals out to steal from you, or they are just bad people who want to destroy other people's stuff for "fun". They especially like to disrupt big businesses.