The Zend_Cache class is part of the Zend Framework and is used (as its name suggests) to cache things. This can be anything from the front end browser output to the outcome of a complex calculation or even the results of database queries. Zend_Cache is an enormous topic, not just how the class works, but what the best practices are for caching.
The best place to start with caching is one of the simpler topics of caching database queries, from this point you can work on understanding other caching functionality. Normally, a call to a database table in Zend Framework might be done like this.