Static RAM ('static random-access memory') is a simple, high-performance memory technology. Unlike dynamic RAM (DRAM), it does not need to be refreshed, making it simpler to use (at the hardware level). The lack of refresh also means that it has a constant access time - there is never a need to wait for a refresh cycle to complete.
It consists of an array of latches, one per bit. The usual analog circuit for those latches contains two transistors and two resistors per bit; it thus requires more transistors per bit than DRAM. It is also volatile, like DRAM, and loses its contents if power is cut off.
It is now used for a variety of purposes in computers, including CPU caches, writeable microcode, disk buffers, bit-mapped displays, etc - anyplace where speed and a guaranteed fixed access time are critical. In the early days of personal computers, it was sometimes used for main memory, but not any more, since it is more expensive per bit than DRAM.