"Distributed" or "grid" computing in general is a special type of parallel computing, it is advanced in the means of using distributed computing.

supercomputers allows both parallel and distributed computing

Serial computing is executing a task on one computer. Distributed computing is executing a task on multiple computers at the same time.

Parallel computing and distributed computing are ways of exploiting parallelism in computing to achieve higher performance. Multiple processing elements are used to solve a problem, either to have it done faster or to have a larger size problem been solved. To state simply, if the processing elements share the memory, it is called parallel computing, other wise it is called distributed computing. Some have opinion that distributed computing is a special form of parallel computing.

HIGH AVAILABILITY provided by Fault Tolerant Computing Systems & Networks which might be geographically distributed and thus overcomes Single Point of Failurein mission critical applications is the main goal of Distributed Computing.