According to Wikipedia, a web cache is a mechanism for the temporary storage (caching) of web documents, such as HTML pages and images, to reduce bandwidth usage, server load, and perceived lag. A web cache stores copies of documents passing through it. In other words, when you click on a link, the computer will search the cache of saved pages and load that in preference to downloading it all over again.
The downside of that is that if there is an error or technical problem, the cached file will retain that error even when the issues have been resolved. It is therefore necessary to periodically clear your cache and your download history, particularly if there have been some technical ‘gremlins’ at work.
To clear your cache, just click CTRL-SHIFT-DEL (i.e. hold down the Control, Shift and Delete keys all at once. On a Mac, the keystrokes are COMMAND-SHIFT-DEL, AND you will see the following pop-up:
In Internet Explorer:
In Mozilla Firefox:
In Safari, the instructions are different – on the History Menu, select Clear History.
Make sure you have selected the options as indicated in each of these images, and click as shown. To complete the process you may have to reboot your computer (turn it off and on again), even if it doesn’t ask you to.