Every time you access the Internet and go to a web address you computer will store the DNS record in its cache. If you aren’t fully sure what DNS is, wikipedia does a great job on explaining the concept. Sometimes this DNS cache gets a bit old and produces errors. This was the case when I switched from Blogger to WordPress on this site. It was bringing up the old DNS records and had my banging my head against the wall.
It clear the cache in Leopard type the following into Terminal:
If you are in Tiger type the following:
Your cache will be clear. It may take a a tiny bit longer to pull up webpages but each request should be correct. I don’t recommend you constantly flush your cache as it will probably do more harm than good. Once a month or less is recommend.
If you still have DNS problems (eg its bringing up the wrong page) I recommend you switching your router on and off, or clearing the cache in your router. These units also have DNS caches and could be another cause of your problems.
If you want to find out more about DNS and all its related workings I recommend the DNS for Dummies these books are always useful and are really well written.