A system is said to be a parallel system in which multiple processor have direct access to shared memory which forms a common address space. 

Parallel systems deal with the simultaneous use of multiple computer resources that can include a single computer with multiple processors. a number of computers connected by a network to form a parallel processing cluster or a combination of both. 

A parallel database system seeks to improve performance through parallelisation of various operations, such as loading data, building indexes and evaluating queries. 

Although data may be stored in a distributed fashion, the distribution is governed solely by performance considerations. Parallel databases improve processing and input/output speeds by using multiple CPUs and disks in parallel. Centralized and client-server database systems are not powerful enough to handle such applications.

In parallel processing, many operations are performed simultaneously, as opposed to serial processing, in which, the computational steps am performed sequentially. 

The goal of parallel database systems is usually to ensure that the database system can continue to perform at an acceptable speed, even as the size of the database and the number of transactions increases. Increasing the capacity  of the system by increasing the parallelism provides a smoother path for growth for an enterprise that does replacing a centralized system by a faster machine.