On a MAC: If you are using the Safari Web browser, choose “Empty Cache…” from the Safari menu.
Tips & Warnings
- The cache keeps copies of recently viewed Web pages on your hard disk. Instead of reloading a page every time, the browser just uses the cached version, which is very fast. (However, it also won’t include any material added to the page since you last viewed it.) When the cache gets large, it can cause performance problems. But if you delete the cache completely, your computer will have to reload every Web page in a session, which might also slow things down. (from ehow)