I like the definition used in Wikipedia: "a cache is a temporary storage area where often accessed data can be stored for quick access". The idea is to get 'often accessed data' from a database and store it in memory (RAM or as a file in your local file system). [...] Resources are limited on systems and to take advantage of your resources, you need to make sure time isn't spent on tasks that could be handled better elsewhere.
He talks about the Zend_Cache component and what sort of features it includes like the different caching methods (file, SQLite, mamcache, etc) and the different frontends your application can use (output caching, class caching, file caching, etc). He sets up a sample environment and shows how to cache a variable to a file, database results into a file, caching an entire page and how to clear out the cached items.