The various levels of CPU cache are fast access memory built on to teh CPU die. Different CPU's have different levels/amounts of built in cache. As a general rule, the higher level the cache is, the more of it exists, but the slower access time is as well.
In todays CPU's, you typically have 3 levels of built in CPU cache. In this case, Main Memory would sometimes be considered the L4 cache. [I still have a lot of books that refer to the L3 cache as main memory...]