A type of computer virus that will copy its own code over the host computer system’s file data, which destroys the original program. After your computer system has been cleaned using an antivirus program, users will need to install the original program again.
See The Difference Between a Virus, Worm and Trojan Horse in the Did You Know? section of Webopedia.
Also see Preventative Maintenance Tips for Your PC in the Did You Know? section of Webopedia.