Joomla caching
Caching is the mechanism that offers a user a stored view of (part of) a web page so that is does not have to be pulled from the database. This usually speeds up the load time of a web page, but the disadvantage could be that a user looks at a stored view, while the actual page has been changed in the mean time. Whether or not this is a problem depends per site. When you are busy developing a site, making a lot of changes, it is also probably best to leave caching switched off. Finally, there could be functionality that does not work with caching at all, like some webshop extensions (Virtuemart, Hikashop a.o.).
For this reason, a default Joomla installation ships with caching switched off. If caching can be used, the Joomla core offers a number of possibilities to use caching, in multiple locations:
Global Configuration - System tab - Cache Settings
Joomla offers 3 possibilities to use caching here:
0 - Off
1- Conservative caching
2- Progressive caching
Progressive caching can cause unexpected issues in rare cases, so it is advised to use the conservative option. This turns on caching for parts of pages.
The Cache Handler can only be set to "File". Joomla actually offers more options, but it takes advanced users to enable this, with additional plugins.
What is worth looking at is Cache Time. Default is 15 minutes, but for sites that are not updated often, you could set this much higher.
System - Cache plugin
In the Plugin Manager, look up the "System - Cache" plugin and enable it. This plugin switches on caching for the whole page being accessed. Especially this plugin can speed up your site considerably.
Optionally you can switch on the "Use browser caching" option, which could be usefull for sites where users ofter return to a page that they already accessed within a browsing session.
Module caching
On a per module basis, you can override the Global setting for cache. You can only switch it off when on a global level it is switched off, not the other way around.
Joomla extensions for Caching
For specific purposes, you can install a caching-extension from the Joomla Extension Directory. This can be especially usefull if parts of your site need to be cached, but others not, requiring a per menu item level of caching. Or maybe you need to have different levels of caching per browser, or mobile devices.
An excellent example of such an extension is Jot-Cache.