There are a few types of caching. (Unless you're Google), your web server will probably have to load the page from a file. Most modern operating systems, if they have room in the ram, cache all files (not just HTML/web files) in ram after reading them, to speed up future reads. This is a type of caching. If a page has to process a big script/CGI executable, if the input variables are all the same it might cache the output. Usually caching will just speed up a single object, unless, say, it just has to put a user's username on every page of a site, for example (it might cache a database query).
