What is Distributed Computing

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.

Cluster Computing:

A computer cluster is a group of loosely or tightly coupled computers that work together closely so that in many respects it can be viewed as though it were a single computer.

1. Better performance and availability and more cost‐ effectiveness over single computer with same capabilities
2. Characteristics:
3. Loosely / tightly coupled computers
4. Centralized Job management & scheduling 
5. Coined in 1987

These clusters are designed to provide uninterrupted availability of data or services (typically web services) to the end-user community.
If a node fails, the service can be restored without affecting the availability of the services provided by the cluster. While the application will still be available, there will be a performance drop due to the missing node.
The purpose of these clusters is to ensure that a single instance of an application is only ever running on one cluster member at a time but if and when that cluster member is no longer available, the application will failover to another cluster member.

Grid Computing:

grid is a collection of resources owned by multiple organizations that is coordinated to allow them to solve a common problem Characteristics

1. Loosely coupled computers
2. Distributed Job management & scheduling 
3. Originated early 1990s