Domain Name Service, it's what allows your computer to find other computers; and in my experience, it's always broken on Linux.
EDIT: NOT HAPPY WITH CloudAtCost
I'm using CloudAtCost to host my server, I am 100% satisfied with the service, but the version on Ubuntu I was given (12.04) is so bare bones it's not even funny. I have to manually add nameserver 22.214.171.124 every time becase the DHCP client resets the /etc/resolv.conf file every reboot. I tried fixing it by write-protecting the file but all that did was break DNS for good.