DNS
The Quick Version
The internet's address book.
How it actually works
DNS (Domain Name System) translates a domain name into the address of a server. When you point a domain at your host you edit DNS records — A records point at an IP address, CNAME records point at another name.
Why does it matter?
Almost every 'my domain isn't working' problem is a DNS record that is wrong or has not propagated yet.
Example
CNAME www → cname.yourhost.com