A domain name server (DNS) is like the phone-book of the internet. Every time you enter a webpage in your browser, it looks up a DNS to find out where the website is before it goes there. Open DNS and google offer a free DNS service, but how do you know if it’s any faster that the one you have?
Your ISP will provide you with some numbers to put in your System Preferences/ Network / DNS Server. It’s probably best not to touch these, but here is how to find the best settings if you want to change them.

Download and run this program called namebench

It will give you a graph like this of the various DNS speeds.

I found out that my DNS (internode) is much faster than google and OpenDNS, so I’ll stick with them.