All communication on the Web is carried out among a set of computers that are interconnected by a computer network. Web technology can be used across an intranet (a network within a company or organization) or across the global Internet. As with all communications among computers, computers that comprise the Web employ two types of software: client and server (see Client/Server Architecture). To make information available, a computer runs a server program. To obtain and display information from a server, a computer user runs a client program. The client contacts a server to request information; the server responds by sending a copy of the requested information. To ensure that the exchange is meaningful, the client and server programs must follow a communication protocol, a set of rules that the two programs use to talk to one another. Like a language, a protocol specifies both the form and meaning of each possible message.
In principle, any computer can run a client or a server. In practice, however, large, powerful computers are usually chosen to run server software, and small personal computers (PCs) are sufficient to run client software. Powerful computers are chosen for server software because they must be able to handle requests for information from millions of people and do so quickly so that users who request information from the server will not experience long delays. PCs, however, are used by a single person to request a Web page. After a user makes a request, the user waits for the information to be displayed. Thus, the client program running on a user's computer only needs to handle one activity at a time. A server, however, must handle simultaneous requests from many clients, possibly millions.
The difference between the Web and the Internet is similar to the difference between a trucking service and a highway system. The Internet corresponds to a highway that allows traffic to flow between computers, and the Web corresponds to a service that uses the highway to move information from one computer to another. Confusion about the difference between the Web and the Internet has arisen because the Web has become extremely popular and currently accounts for the majority of Internet traffic. However, other services also use the Internet to carry their traffic. For example, the Internet's electronic mail service permits users to send and receive textual messages, and the file transfer service allows a user to transfer a copy of a file from one computer to another.
Although many services use the Internet to carry data from one computer to another, each service follows a separate set of rules that define the messages used in the exchange. The Web uses the HyperText Transfer Protocol (HTTP), electronic mail uses the Simple Mail Transfer Protocol (SMTP), and file transfer uses the File Transfer Protocol (FTP). The application programs that users run to access the Internet often blur the distinction among these services. For example, an application program that can send e-mail also allows a user to transfer the contents of a file, and an application program used to access the Web also allows the user to process e-mail.