Nslookup is a program that lets an Internet server administrator or any computer user enter a host name (for example, "good2use.com") and find out the corresponding IP address. It will also do reverse name lookup and find the host name for an IP address you specify.
For example, if you entered "good2use.com" you would receive as a response an IP address, which happens to be :

70.84.250.212

Nslookup sends a domain name query packet to a designated (or defaulted) domain name system (DNS) server.
Depending on the system you are using, the default may be the local DNS name server at your service provider, some intermediate name server, or the root server system for the entire domain name system hierarchy.

Using the Linux and possibly other versions of nslookup, you can locate other information associated with the host name or IP address, such as associated mail services. nslookup is included with some Unix-based operating systems and in later Windows systems.
In Windows XP, the command can be entered on the "Command prompt" screen, click Start, Run and type CMD.