Displays information about a nickname. This includes the host that they are logged in from, the server they are logged into, the public rooms they are in, private rooms you are both in, and the number of seconds that have passed since they last communicated with the server. There may be additional information depending upon what fields they have set.
Example:
/whois webmaster
Whois is generally the first command you would use on people to learn something about them. For related commands see who, whowas, and names.