Your browser has a folder in which certain items that have been downloaded are stored for future use. Graphic images, photo's, and even entire web pages are saved or cached. When visiting a webpage your browser checks if a copy of the files on the page is in its cache already.
There are some situtations when bypassing your browser's cache is prefered. Just clicking the refresh button (or hitting F5) won't be sufficient in this case, because this reloads the webpage while still using the old files from the cache. You need to refresh your cache first! Refreshyourcache.com contains step by step guides to clear your browser's cache forcing it to download all latest data from a website!