A virus is a malicious program or piece of code created to cause problems on computers. Viruses piggyback on top of programs, zip files and documents to infect vulnerable computers. For instance, a virus may be hidden inside a file attached to an email message. Opening the email attachment activates the virus, which may cause your computer to malfunction or delete valuable data. If you send the infected file to someone else, the virus may spread to that person's computer.
You may have also heard of Trojan horses or worms. These are simply different kinds of computer viruses. A worm typically will make a copy of itself in the form of a file and spread through systems that are connected, whether it is the Internet or a company network, and ultimately causing damage to each infected system.
A Trojan horse masks itself as a program that appears to be normal, but when opened it can cause damage to the data or security of the system it infects.
Antivirus software protects your computer from these malicious viruses.