A distributed computing system is basically a collection of processors interconnected by a communication network in which each processor has its own local memory and other peripherals, and the communication between any two processors of the system takes place by message passing over the communication network.
Computer architectures consisting of interconnected, multiple processors are basically of two types:
1). Tightly coupled system
2). Loosely coupled system
Tightly coupled systems
In these systems, there is a single system wide primary memory (address space) that is shared by all the processors . Usually tightly coupled systems are referred to as parallel processing systems.
Loosely coupled systems
In these systems, the processors do not share memory, and each processor has its own local memory .Loosely coupled systems are referred to as distributed computing systems, or simply distributed systems
For details please download the presentation attached below on Distributed Computing