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Computer - A programmable machine. The two principal characteristics of a computer are:
It responds to a specific set of instructions in a well-defined manner.
It can execute a prerecorded list of instructions (a program).
Modern computers are electronic and digital. The actual machinery -- wires, transistors, and circuits -- is called hardware; the instructions and data are called software.

All general-purpose computers require the following hardware components:

memory : Enables a computer to store, at least temporarily, data and programs.
mass storage device : Allows a computer to permanently retain large amounts of data. Common mass storage devices include disk drives and tape drives.
input device : Usually a keyboard and mouse, the input device is the conduit through which data and instructions enter a computer.
output device : A display screen, printer, or other device that lets you see what the computer has accomplished.
central processing unit (CPU): The heart of the computer, this is the component that actually executes instructions.
In addition to these components, many others make it possible for the basic components to work together efficiently. For example, every computer requires a bus that transmits data from one part of the computer to another.

Computers can be generally classified by size and power as follows, though there is considerable overlap:

personal computer : A small, single-user computer based on a microprocessor. In addition to the microprocessor, a personal computer has a keyboard for entering data, a monitor for displaying information, and a storage device for saving data.
workstation : A powerful, single-user computer. A workstation is like a personal computer, but it has a more powerful microprocessor and a higher-quality monitor.
minicomputer : A multi-user computer capable of supporting from 10 to hundreds of users simultaneously.
mainframe : A powerful multi-user computer capable of supporting many hundreds or thousands of users simultaneously.
supercomputer : An extremely fast computer that can perform hundreds of millions of instructions per second.

Network - A group of two or more computer systems linked together. There are many types of computer networks, including:
local-area networks (LANs) : The computers are geographically close together (that is, in the same building).
wide-area networks (WANs) : The computers are farther apart and are connected by telephone lines or radio waves.
campus-area networks (CANs): The computers are within a limited geographic area, such as a campus or military base.
metropolitan-area networks MANs): A data network designed for a town or city.
home-area networks (HANs): A network contained within a user's home that connects a person's digital devices.
In addition to these types, the following characteristics are also used to categorize different types of networks:

topology : The geometric arrangement of a computer system. Common topologies include a bus, star, and ring. See the Network topology diagrams in the Quick Reference section of Webopedia.
protocol : The protocol defines a common set of rules and signals that computers on the network use to communicate. One of the most popular protocols for LANs is called Ethernet. Another popular LAN protocol for PCs is the IBM token-ring network .
architecture : Networks can be broadly classified as using either a peer-to-peer or client/server architecture.
Computers on a network are sometimes called nodes. Computers and devices that allocate resources for a network are called servers.

Hosting - A Hosting Services Provider (HSP) is an application service provider dedicated to providing hosting services. Typically operates a Web server farm , either at a data center or colocation facility.

Computer Virus - A program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. 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.

Since 1987, when a virus infected ARPANET , a large network used by the Defense Department and many universities, many antivirus programs have become available. These programs periodically check your computer system for the best-known types of viruses.

Some people distinguish between general viruses and worms. A worm is a special type of virus that can replicate itself and use memory, but cannot attach itself to other programs.