DRAM starts a second memory reference before the first is done. This is done by adding more pipelines. Both of these chips are asynchronous, and are driven by a single clock. These cannot be changed or erased. PROMs have an array of fuses, which are blown to create specific paths, like the Programmable Logic Arrays.
Flash memory is block erasable and rewritable.
They are used as film for storing pictures in digital cameras. However, they can only be rewritten about 10,000 times.