Cache Tab:

Caching is a means to improve the speed at which Joomla! shows its Content to a visitor when they select a Content Item. In essence Joomla! creates a copy of the database Content Items on the server hard disk. When a particular Content Item is requested by a visitor Joomla! will check the cache for the item and display it much quicker than if it had to send the request to the MySQL database.

Caching: Select Yes to allow caching of certain elements within the Joomla! web site. Caching will generally improve the performance of the web site.

Cache Folder: This is the path to, and directory where, Joomla! will store the cache files. This directory must be writable for caching to be successfully implemented. 

Cache Time: Edit the maximum length of time, in seconds, for a cache file to be stored before it is refreshed. If the Content on the web site is continually being updated as would be the case for example with an active forum, then a more frequent refresh rate would be justified. If, however, the web site is orientated more towards static Content, then setting a refresh rate of 12 or even 24 hours may be in order.

NOTE: Just remember cache refresh time is in seconds so 12 hours would be 12*60*60=43,200 seconds.