Computer Viruses
A computer virus is a small piece of software that piggybacks on real programs and replicates itself. For example, a virus might attach itself to a program such as an Excel document.
Each time the document is opened, the virus runs as well and has the chance to reproduce (by attaching to other programs) or perform any number of destructive actions to user data.
An e-mail virus moves around in e-mail messages, and usually replicates itself by automatically mailing itself to dozens of people in the victim's e-mail address book. There are different types of computer viruses, among which are:
Worms are small pieces of software that use computer networks and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well.
Trojan horses are computer programs that claim to do one thing (such as a game or screen saver) but instead does damage when you run it (it may erase your hard disk, allow an outside user access to your system, or more). Trojan horses have no way to replicate automatically and rely on the user to run them.
Tools