SRAM is static, which means that it can retain its data using a very low voltage electrical charge and doesn't need to be refreshed. As long as it has power, SRAM holds its charge and contents. SRAM also has very fast access times, in the range of 15 to 20ns (compared to the 50 to 120ns of DRAM).
The following table contrasts SRAM to DRAM in terms of their general characteristics:
|Speed||Slow and must be constantly refreshed||Fast and doesn't require refreshing|
DRAM is used for main memory due to its small size and low cost while SRAM, being fast, is used as cache; its operating speed is nearer that of the processor than DRAM.
SRAM is available as either synchronous or asynchronous. Synchronous SRAM uses the system clock to coordinate its signals with the CPU, and asynchronous doesn't.