A computer virus is generally defined as any sort of destructive computer program, though the term is usually reserved for the most dangerous ones. There are many ways in which viruses can be spread. A virus can be introduced to networked computers thereby infecting every computer on the network or by sharing disks between computers. As more home users now have access to modems, bulletin board systems where users may download software have increasingly become the target of viruses. Viruses cause damage by either attacking another file or by simply filling up the computer's memory or by using up the computer's processor power.
Viruses are, in essence, self-replicating.
People in the computer industry do not agree on the distinctions between worms and viruses. Regardless, a worm is a program specifically designed to move through networks. A worm may have constructive purposes, such as to find machines with free resources that could be more efficiently used, but usually a worm is used to disable or slow down computers. These programs move of their own volition based upon stored knowledge of the network structure. These viruses hide inside another seemingly harmless program and once the Trojan Horse program is used on the computer system, the virus spreads.