DRAM stands for dynamic random access memory. DRAM is a technical term for a type of random access memory (RAM) that can retain its contents only for a very brief period (measured in milliseconds) and must therefore be continually refreshed by reading its contents at short intervals.
Despite the fact that DRAM requires extra REFRESH circuitry to be designed into the computer's PROCESSOR BUS, it has until recently been the most widely used type of memory because it is so much cheaper to manufacture than non-volatile STATIC RAM. DRAM is the most common type of memory found on SIMMs.
The CELLS that store the bits in a DRAM chip are simple single-transistor CAPACITORS (whose charge leaks away, hence the refresh requirement) and more of them can be packed onto a single chip than the multi-transistor cells used in STATIC RAM, reducing the cost per megabyte. DRAM is only now falling out of favour - on the grounds of speed rather than cost, because it cannot be read fast enough to keep up with the latest generations of microprocessor.