DRAM stands for Dynamic Random Access Memory. It is a type of semiconductor memory in which the memory is stored in the form of a charge. Each memory cell in a DRAM is made of a transistor and a capacitor. The data is stored in the capacitor. Capacitors loose charge due to leakage and hence DRAM's are volatile devices. To keep the data in the memory, the device must be regularly refreshed whereas SRAM is static, so it will retain a value as long as power is supplied. SRAM is typically faster than DRAM since it doesn't have refresh cycles. Since each SRAM memory cell is comprised of 6 Transistors unlike a DRAM memory cell, which is comprised of 1 Transistor and 1 Capacitor, the cost per memory cell is far greater in an SRAM compared to a DRAM. With similar reasoning, it can be asserted that DRAMs come in larger densities than SRAMs given a fixed area. Example : SRAMs are used in Caches because of higher speed and DRAMs are used for main memory in a PC because of higher densities.