There are two types of RAM:
Static RAM (SRAM) retains its contents as long as electrical power is applied to the chip. If the power even if lost temporarily, its contents will be lost forever. They are faster than DRAM but are expensive. SRAM is used only where access speed is extremely important.
Dynamic RAM (DRAM) has an extremely short data lifetime-typically about four milliseconds. They are effective whenever large amounts of RAM are required.
Apart from SRAM and DRAM, there are EDO RAM, VRAM (Designed for graphics), SDRAM (next level of DRAM), DDR RAM (Next level of SDRAM) and RDRAM.
