Difference betwwen shared memory and distributed memory

Distributed Shared Memory (DSM)

In DSM the different physical memories are logically shared over a large address space( virtual memory). So the processes going on accesses the physical memory through these logically shared address space. DSM have no physical shared memory. The shared memory model provides a virtual address space shared between all nodes. The term "shared" does not mean that there is a single centralized memory but "shared" essentially means that the address space is shared (same physical address on two processors refers to the same location in memory).

Shared Memory (SM)

In computer science, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Shared memory is an efficient means of passing data between programs. Depending on context, programs may run on a single processor or on multiple separate processors.
In computer software, shared memory is either
-a method of inter-process communication (IPC), i.e. a way of exchanging data between programs running at the same time. One process will create an area in RAM which other processes can access;
-a method of conserving memory space by directing accesses to what would ordinarily be copies of a piece of data to a single instance instead, by using virtual memory mappings.