Which is the fastest accessible memory? What it is made-up of?

Cache is the fastest accessible memory of a computer system. It's access speed is in the order of a few nanoseconds. The CPU can only directly fetch instructions and data from cache memory, located directly on the processor chip. Cache memory must be loaded in from the main system memory It is volatile and expensive, so the typical cache size is in the order of megabytes. 

Cache is a portion of memory made of high-speed static RAM (SRAM). Cache memory is memory actually embedded inside the CPU. In fact, there are several sub-levels of cache memory (termed L1, L2, L3) all with slightly increasing speeds.