Over the years different models for performing computation has been developed, (e.g., batch processing, time-sharing, etc.). The two most common forms of application computation: stand-alone and distributed computing will be briefly described here.
The majority of computing, for most of the existance of computers, has been performed on single-computers, one program executing at a time. A stand-alone application program solves a problem by itself. It performs all the necessary computation using only the local resources available to it. Since it is not connected to any network or other machine, a standalone machine shares no data, nor any resources with any other computer.
Networking came about because of the need for distributed data. If you consider an airline booking system, thousands of check-in personnel, airline reservationists across the world needs to gain access to the same data, the schedules and seats of the aircrafts an airline provides.
Simply put, "networking is about making connections." Just as a professional can "network" with other people to share ideas, and be more productive, a computer can network with other computers to share data, such as airline schedules and occupancy, and share resources such as printers, modems, and even hard disks. An apt metaphor for a networked computing system is the interconnected cells of the human brain. Similar to the human brain, individual cells can perform processing, but the interconnections and sharing of data between the brains provide even more capabilities.
The required access to a pool of data saw the growth of operating systems into large centralized mainframe configured computers. In this configuration, one large computer provided most of the data and the computing power, and several directly connected hard-wired terminal interfaces provided control devices, (for entering programs or issuing commands), for the computer system. These terminal interfaces could be remotely located in another room, or for an application such as an airline booking system, across the nation as long as they were connected to the mainframe through dedicated lines.
The advent of micro-computers brought about the decline of centralized mainframe computing and made possible the currently popular use of the client-server (C-S) model, currently the dominant form of computing. In the C-S model, client applications send messages requesting services from server applications, (hence the name). The decomposition of monolithic operating systems into C-S systems provided the background research for the application of the C-S model to networks. Clients tend to be small programs that leave the bulk of computing and policy making up to server programs dedicated to "serving" requests for data. Although a computer can be dedicated to be a server or as a client, modern operating systems and computing platforms are multi-tasking and can support both modes simultaneously. That is, a computer can run several server applications at the same time it runs several client applications.
Currently, there are several types of networks. Local Area Networks (LANs) are made up of computers physically located within the same premise, usually a room or perhaps building. The actual networking mechanism that connects the computers and hardware such as printers that make up a LAN are dedicated exclusively to supporting the LAN. A Wide Area Network, WAN, is much like a LAN except that it is distributed over a larger area, such as a university campus. A WAN can also interconnect several LANs.
The Internet, of which the World Wide Web (WWW) is but a part, is truly distributed. The connection lines that support the Internet are varied, including modems and telephone lines, video cables, high speed T1 lines, fiber optics, and even satellite uplinks.
The World Wide Web is a very distributed form of computing. When you click on a hyperlink, the browser sends a request to a computer somewhere in the world for a certain file. The Web server on the remote computer responds to the request and returns the appropriate document to the browser. The browser then renders the document to the screen. It is this C-S model that requires you to be connected to the internet for Web browsing.
The latest type of network is the Intranet. Intranet technology was developed to provide the security of dedicated networks, but over the cheap backbone of the Internet. It uses the same infrastructure of connected computer relays and connection lines as the internet, but employs special protocols, (think of it as cyphers or languages), and encryptions to simulate a dedicated network.