Nameservers tell Internet Browsers where to go to find the files that make up your web site.
If you think of a domain name as the name of your site, your nameservers are the zip code.
When you change your nameservers the information has to be passed or "propogated" to all the major Internet hubs across the planet. This process usually takes a day but at Wilmington Web we know how to action the change so that, from your visitors point of view, the change is practically invisible.
For more information see our beginners guide to Web Hosting.