What is distributed computing

A distributed computer system consists of multiple software components that are on multiple computers, but run as a single system. The computers that are in a distributed system can be physically close together and connected by a local network, or they can be geographically distant and connected by a wide area network. A distributed system can consist of any number of possible configurations, such as mainframes, personal computers, workstations, minicomputers, and so on. The goal of distributed computing is to make such a network work as a single computer.

Distributed systems offer many benefits over centralized systems, including the following:
The system can easily be expanded by adding more machines as needed.
Several machines can provide the same services, so if one is unavailable, work does not stop. Additionally, because many smaller machines can be used, this redundancy does not need to be prohibitively expensive.

Distributed computing systems can run on hardware that is provided by many vendors, and can use a variety of standards-based software components. Such systems are independent of the underlying software. They can run on various operating systems, and can use various communications protocols. Some hardware might use UNIX or Linux as the operating system, while other hardware might use Windows operating systems. For intermachine communications, this hardware can use SNA or TCP/IP on Ethernet or Token Ring.

You can organize software to run on distributed systems by separating functions into two parts: clients and servers. This is described in The client/server model. A common design of client/server systems uses three tiers, as described in Three-tiered client/server architecture.