The real and virtual memory category is pretty simple. Real memory is the actual memory chips. Virtual memory is not real but provides the appearance of a lot more real memory. Some operating systems manage this better than others, but I digress.

Shared memory segments reside in real memory (we hope), and a process creates them using the shmget call. Most DBAs quickly figure this out once they try to start an Oracle instance and receive a shmget-related message, and then need to track down what the heck just happened.

What is unique about shared memory segments is that multiple processes can access the same piece of memory. The alternative would be to place the common data on disk. What a locking nightmare that would be! On Linux and Unix systems, Oracle's SGA is stored in shared memory segments.