Public DNS services like OpenDNS or Google DNS may offer more faster lookups than your ISP DNS server of your ISP but in some cases, you may get much better download speeds if you use your ISP settings.
The job of a DNS Service, like Google DNS or OpenDNS, is to translate web domain names (like google.com) into IP addresses. It is therefore important that you choose the fastest DNS Server for your computer since it is used every time you visit a new site in the browser.
This short video explains the concept of DNS (Domain Name Server) and how DNS works in simple English.
This video tutorial describes how you can setup Google DNS servers on your own computer or wireless router. Instructions are available for Windows XP, Vista, Windows 7 and wireless routers.