Computer viruses are actually small programs that have the ability to replicate themselves and then spread throughout your system in many ways. This is usually done without a user’s knowledge. Such programs may copy themselves to a shared network resources or mail themselves to E-Mail Contacts that are stored in your address book.
After a Virus has infected your system, it will usually trigger itself in some sort of fashion. It can launch annoying pop up windows that repeatedly place messages on your screen. It can deny you access to your data files and even change your system configuration settings. In advanced stages of infection, a virus can cause major damage to your system as it can corrupt your files and render them unreadable. It can even start deleting files as well. A virus doesn’t always have to trigger itself upon infection. It can actually be programmed to launch at a specific time or on a specific date so that there will be no obvious sign of system infection. A virus will usually disguise itself and may even attach itself to another file, so as to trick a user into running it.
A Worm is a small piece of software that uses 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. The term worm was coined by John Brunner, a science fiction writer, in his 1975 novel Shockwave Rider. The hero, a talented programmer, created self-replicating computer programs that tunneled their way through a worldwide network