Browsers cache (temporarily store) lots of files used to display websites you've recently visited. This can be difficult when you're developing websites as you can often be looking at old versions of parts of the page. Notoriously difficult to refresh are CSS (styles), javascript (behaviours) and Flash movies. There are lots of tricks - holding the Shift key while pressing reload, appending spurious random magic query strings. But often the most reliable is using the browsers built-in tools. I'm maintaining a list here.

Macintosh

• Firefox > preferences > advanced > network > offline storage > clear now
• Chrome > clear browsing data > empty cache
• Safari > empty cache