A cache is a storage place where web pages can be held for quick access by the server, without the need to build them from scratch each time. Obviously, this can speed up a site significantly if it is providing pages that require large data retrievals each time. The problem is that a cached page serves an old version of the page, and not the most recent one. This is no problem if you are working with pages which only change once in a while, but if it's important that users are able to see the latest version of a page each time it is requested, then you should not use caching.
So when do we not need caching? For a start, there is no need to use it while you are building your site, because any changes you make while you are setting things up may not display until the cache is flushed—this can lead to no small amount of confusion when you are expecting a change to manifest itself on a page. Accordingly, you should only really enable page caching just before your...