CPU Cache

Your CPU is fast. Blazingly fast. On the order of millions or billions of operations per second (megahertz... gigahertz...).

This mind-boggingly fast CPU is just too fast for hard drives or even RAM to keep up. If the CPU always had to wait for information from the RAM, the CPU would be idling most of the time and not doing useful tasks (like playing music in iTunes while I'm writing this).

The solution to this problem was to create another layer of memory that is faster for the CPU to talk to but is smaller than the RAM or hard drive. This is the CPU cache.

See the Wikipedia article on CPU caches for more information (and technical details)