Almost every website is comprised of dozens, sometimes hundreds or even thousands, of small files. Since many portions of websites do not change regularly, your web browser stores (caches) these files on your computer's hard-drive. This makes it possible for the website to load faster the next time you visit.
Caching files works until a change is made to one or more of the Website files. Since cached files often deal with how a page is displayed, updates made to the page may not display. It is also important to know that “refreshing” a page may not clear all cached files.
You may also want to regularly clear your cache, but don't want to go through those steps every time.