"Cache is a component that transparently stores data so that future requests for that data can be served faster." Wikipedia
So basically, when you view a website, your browser cache stores what you are viewing on your hard drive to make it load much quicker next time you visit the same page. Unfortunately it means you are sometimes viewing old data. You need to "empty" or "clear" cache, then refresh/reload the page to view the most recent version of the website/page.
To empty/clear cache:
Safari:
it is just in the drop down menu entitled "safari".
Firefox it is in "Preferences > Network > Offline Storage > Clear Now".
Internet Explorer click "Internet Options > General > Temporary Internet Files > Delete".
