The /etc/resolv.conf file is where CentOS looks to resolve hostnames and domainnames that are not in the /etc/hosts file.
You can specify your nameservers as well as your search path and some other options in here.
Lets look at an example resolv.conf with comments telling you what each line does.
search localdomain dev.centoshowtos.org centoshowtos.org
options attempts:1 rotate timeout:1
It’s also worth noting that this file is overwritten by the dhcp client if you do not have a statically assigned address. If you obtain your IP via dhcp, you want to set up the dhcp server to handout these options.