As a rule, the CPU cache is basically REALLY fast access high speed memory. Each level is slightly larger then the last, but also slightly slower. In a multiple-core environment, one or multiple levels of cache may be dedicated to either a specific core, or for the entire CPU as a whole.
As a general rule: L1 Cache: 2-3 clcok cycle access L2 Cache: ~10 clockcycle access L3 Cache: ~20-30 clockcycle access Main Memory: ~100 clock cycle access; > 100,000 cycle access if a new page must be loaded. (Hence why more RAM increases speed: Fewer page faults).