Basically - if someone types in Yahoo.com, it is the DNS that says to your computer "go here to load up Yahoo.com"
Web hosting is basically a bunch of servers (powerful computers) all connected to each other. Not just a few - tens of millions of servers connected to each other. So if I type in Yahoo.com - how do you know which server to access? DNS!
The DNS for Yahoo.com is what points you to the right server to access Yahoo.com.
Usually the DNS for a domain is the host they are using. So if you are using TheHosted, you will use the DNS provided by them (something like ns1.thehosted.com or dns1.hosted.com)
