Usually this due to the Cache Mechanisms.
In this wikipedia article you will see that caching is an important computing feature that is used widely. http://en.wikipedia.org/wiki/Cache_(computing)
When enabled in Joomla, it delivers to the users a stored view of the web page, same as it was previously displayed, so it won't have to ask the database and reload content, thus it can speed up the speed load of a website and save resources on the server.
Joomla provides the following caching levels:
a. Page cache
It stores a full copy of the rendered page. It is controlled by a system plugin : Plugins Manager -> System Cache plugin, and it is disabled by default. It applies only to non-logged in users.
b. View cache
It stores part of the page. It is also disabled by default and can be accessed in the Global configuration -> System.
It provides 2 caching ways: Conservative and Progressive cache.
Progressive caching caches the content for each unique visitor, while Conservative caching delivers the same cached content to all visitors.
There you can also control the time that a cached content should be remained, before it is considered expired and therefore replaced with a new one. If the content updates are frequent you may want to set this to a low value.
c. Module Caching
It also partially stores content of the page, per module. It is controlled from within each module, usually in the advanced section of module settings. It is recommended to be enabled only for modules that contain static content.
When cache is enabled, Joomla stores the cached content inside the Cache folder of its filesystem ( siteroot/cache/ ).
3rd Party Caching Extensions
Apart of the core cache functionality, there are 3rd party extensions that allows more control over how the cache should work on a Joomla site.
Joomla Cache Extensions in JED
The cached content can be deleted/emptied, if you want to remove all cached content and force joomla to reload all content fresh from the database. Sometimes clearing caches, it helps resolving display or other issues, so it's common practice to clear or disable the cache when something doesn't look or work as expected, to ensure that is not a caching issue.
To clear Joomla Cache go at:
System -> Maintenance -> Clear Cache / Purge Expired Caches.
Clear Cache will remove all cached content and the system will have to regenerate/reload it at the next page load. This might make the next load of a page to take a bit more time, since it will also have to create the new cached files, but after the next load the pages will load faster again.
Purge Expired Caches will only remove items that have expired. This will check each cached file by file if it has expired and will delete only these files, therefore if the website is big, it can be resource intensive during the purging process.
Nonumbers Cache Cleaner extension is a system plugin that can be set to clean up caches within specified timeframes, or after "Saving" anything in the backend.