Multiprocessors approaches

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Shared memory multiprocessor: In this model, every CPU has equal access to the entire physical memory, and can read and write individual words using LOAD and STORE instructions. This model may be implemented using physical separated CPUs, multiple cores on a a single CPU, or a combination of the above. While this model sounds simple, actually implementing it is not really so simple and usually involves considerable message passing under the covers. However this message passing is invisible to the programmers.

Message-Passing multicomputer: The CPU-memory pairs are connected by a high-speed interconnect. Each memory is local to a single CPU and can be accessed only by that CPU. The CPUs communicate by sending multiword messages over the interconnect. There is no shared global memory in this design. They are much easier to build than shared-memory but they are harder to program.

Wide area distributed system: Connects complete computer systems over a wide area network, such as the internet, to form a distributed system. Each of theses has its own memory and the systems communicate by message passing. The difference between 2 and 3 is 2 is tightly coupled while 3 is loosely coupled.