Whenever you visit a website, your computer needs to retrieve the IP address for that address. To improve efficiency and speed, your computer will cache the name resolutions, so that if you visit Google a second time, you don't have to ask for the IP address again.

In Windows, if you want to view your DNS cache, the command is:

ipconfig /displaydns

In Mac (Snow Leopard), you can do the same thing by:

dscacheutil -cachedump -entries Host

And you can flush your DNS cache with:

ipconfig /flushdns

in Windows and:

dscacheutil -flushcache

in Mac.