Every browser (Internet explorer or Firefox are just examples of a browser) has a folder in which certain items that have been downloaded are stored (or saved to cache) such as buttons, icons, photo's, and even entire web pages. When visiting a web page your browser checks if a copy of the files on the page is in its cache already. If so, it will save the visitor some downloading time and make the web page load faster by using the files it has already saved in the cache. Some browsers refer to it's cache as Temporary Internet Files.
Normally hitting F5 (on a Mac Cmd+R) - however it is still not refreshing as expected you can hold down your Control key and hit F5 for a hard refresh. This will normally re-download the page from the servers showing the new updated content on the page.
If you are still experiencing problems visit http://www.wikihow.com/Clear-Your-Browser's-Cache for a more detailed description on all major browsers.