http://wiki.answers.com/Q/What_are_the_differences_between_SRAM_and_DRAM
SRAM(Static Random Assessable Memory)-where the word static indicates that it, does not need to be periodically refreshed, as SRAM uses bistable latching circuitry (i.e., flip-flops) to store each bit. Each bit is stored as a voltage.Each memory cell requires six transistors,thus giving chip low density but high speed.However, SRAM is still volatile in the (conventional) sense that data is lost when powered down.
Disadvantages are more expensive and also consumes more power than DRAM.
也正是因为sram的结构原因，所以sram操作时可以异步写(输入地址，对应的值就是flip-flop q端的值)，但却只能同步读（上升沿的时候才可以改变里边flip-flops的值）
In high speed processors (such as Pentium), SRAM is known as cache memory and is included on the processor chip.However high-speed cache memory is also included external to the processor to improve total performance.
DRAM(Dynamic Random Assessable Memory)- Its advantage over SRAM is its structural simplicity: only one transistor (MOSFET gates) and a capacitor (to store a bit as a charge) are required per bit, compared to six transistors in SRAM. This allows DRAM to reach very high density.Also it consumes less power and is even cheaper than SRAM (except when the system size is less than 8 K) .
But the disadvantage is that since it stores bit information as charge which leaks;therfore information needs to be read and written again every few milliseconds.This is known asrefreshing the memory and it requires extra circuitry,adding to the cost of system.