What is cache and what should I buy ?

Caching is a technique to improve a computers performance by providing fast temporary storage between the CPU and system RAM. In typical desktop and laptop systems several layers of caching are used. The fastest cache, the L1 cache, has a small capacity but is very fast. The L1 cache connects the CPU with a second layer of cache, the L2 cache, this in turn is connected to the main system RAM or to a third level of cache, the L3 cache.
What is the best sized cache ?
With caching the short answer is, bigger is better. For a given CPU, RAM and motherboard, increasing the size of cache will improve system performance in a very broad range of tasks.