This file configures the name resolver, specifying the address of your name server (if any) and your domain name. Your domain name is your fully-qualified hostname (if you're a registered machine on the Internet, for example), with the hostname chopped off. That is, if your full hostname is loomer.vpizza.com, your domain name is just vpizza.com.
For example, if your machine is goober.norelco.com, and has a nameserver at the address 188.8.131.52, your /etc/resolv.conf would look like:
You can specify more than one nameserver---each must have a nameserver line of its own in resolv.conf.