Caching is an important principle of computer systems. Information is
normally kept in some storage system (such as main memory). As it is used,
it is copied into a faster storage system—the cache—on a temporary basis.
When we need a particular piece of information, we first check whether it is
in the cache. If it is, we use the information directly from the cache; if it is not,
we use the information from the source, putting a copy in the cache under the
assumption that we will need it again soon.