Resolution is the process of turning a name into an IP address or an IP address back into a name. The process begins with asking the root servers which nameserver to ask about the domain name that is being resolved. The root nameservers will refer you to another name server which will refer you to another name server and so on. Finally you will be refered to the nameserver which is authoritative for the domain name. The authoritative server will look in its zone files and respond with an answer.
DELEGATION
Delegation is the process of distributing the responsibility for resolving names or IP addresses across multiple DNS servers maintained by different organizations. The delegation process is how DNS names, domain names, are resolved.
FORWARD vs. REVERSE
When the process turns a name into an IP address, it is referred to as forward resolution. When an IP address is resolved back into a name the process is called reverse resolution.