Caching is a way to maintain state. There are various caching approaches.
One easy one to think about is data caching. The idea is to load data (state)
from the database once, and save it in memory (in the cache).  The next time
you need that data, instead of taking the time to go to the database you
can just load it form the cache. It's intended to imporve performance. The
one problem is that the data you've stored in the cache is a copy of the
original data, so it could be out of date.

-Brock
DevelopMentor
http://staff.develop.com/ballen