In a nut shell. The most common applications of parallel computing/processing are solving extremly complex problems whithin the science and engineering communities e.g. ... grid computing and internet technology.

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.

The definition of parallel computing is the processing of data many bits at a time as opposed to serial computing which is the processing of data one bit at a time.

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

what is computing resources in information technology.