Created by just a handful of programmers, UNIX was designed to be a small, flexible system used exclusively by programmers. Although it has matured considerably over the years, UNIX still betrays its origins by its cryptic command names and its general lack of user-friendliness. An address to a specific file or location on the Web that all browsers recognize. A URL always begins with an Internet protocol, includes the domain, intermediary directories or folders within the file structure of the Web site, and ends with the filename of the Web page. A program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes.
Most viruses can also replicate themselves.
All computer viruses are manmade. A simple virus that can make a copy of itself over and over again is relatively easy to produce. Even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. An even more dangerous type of virus is one capable of transmitting itself across networks and bypassing security systems. Some people distinguish between general viruses and worms.