What is SRAM and DRAM?

SRAM (static RAM) stores each bit in a flip-flop made of about six transistors. It keeps the data as long as power is on and needs no refresh. It is fast but costly and takes more space, so it is used for cache memory.

DRAM (dynamic RAM) stores each bit as charge on a tiny capacitor with one transistor. The charge leaks, so every row must be refreshed within a few milliseconds. DRAM is slower, cheaper and denser, so it is used for main memory.