Whois is one of the oldest and simplest information servers in the Internet. Whois allows you to lookup someone's e-mail address and other information that a user may be happy to give away, simply by knowning their name.
Originally, it was a purely central server run on the ARPANET for all managers/contacts of networks attached to the ARPANET for the DCA (Defence Communications Agency). [RFC 954]
Basically, a whois server runs on TCP port 43, and awaits simple command lines (in ASCII text, ending with CRLF). The server simply looks up the command line or ``name specification'' in a file (perhaps using fuzzy or soundex matching) and responds, possibly with multiple matches. Whois is for keeping organisation contact information.
Table 2.6: Example of Whois output
Note that each returned entry has a NIC handle, to distinguish it (i.e. act as a unique key).