Cache memory is the fastest storage buffer for the central processing unit of a PC (etym. F. 'cacher' =hide). Uses SRAM (static RAM). Distinguish L1, L2 and L3 cache. A few years ago L2 cache was still on the motherboard, but now it is a part of the CPU chip. L3 cache, rare, may still be external, but already the first CPU with L3 on the chip has arrived.
The difference between the various cache-levels lies in their size and, thereby, their speed. L1 is the fastest and may still be as small as 12 kB. L2 cache of 128 or 256 kB is quite state-of-the-art. L3, if it exists, comes in even bigger sizes and is consequently slower. Typically the cache is used in repeat-type instructions, which are usually abundant[running a PC without cache, which can be done, may slow it down by a factor 100].
Static RAM. Might be a misleading name. A type of RAM (Random Access Memory) that is quicker than dynamic RAM and does not need to be refreshed. Because it is more expensive and holds less data than dynamic RAM, it is used primarily for cache memory. An SRAM cell consists of a minimum of 4 transistors (4T-SRAM),but 6T-SRAMs with 4 NMOS and 2 PMOS transistors are today's standard. A more compact solution is a combination of 4T-SRAM with thin film transistors (TFT) in a layer on top.
Courtesy of the French Energy Agency ADEME, Future Electronics project.