In this article we discussed about the hardware platforms of parallel and distributed simulation technology of interest here contain a potentially large number of processors interconnected through a communication network. In most cases the processor is a general purpose CPU (central processing unit), often identical to those commonly found in personal computers and engineering workstations. The switching network may be as specific as a customized switch for a particular multiprocessor system, or as general as the Internet. This paper presents four categories: Parallel versus Distributed Computers, Shared-Memory Multiprocessors, Distributed-Memory Multicomputers, SIMD Machines and Distributed Computers. This paper is a theoretical paper.