Google Public DNS is a free, global Domain Name System (DNS) resolution service, that you can use as an alternative to your current DNS provider.

Why does DNS matter?

The DNS protocol is an important part of the web’s infrastructure, serving as the Internet’s phone book: every time you visit a website, your computer performs a DNS lookup. Complex pages often require multiple DNS lookups before they start loading, so your computer may be performing hundreds of lookups a day.

More details here
http://code.google.com/speed/public-dns/

UPDATE:

• Video: How to Setup Google DNS on your Computer
• Here’s What That Means