Usage: WHOIS <nick>
Displays information about a nickname. This includes the (hashed) host that they are logged in from, the server they are logged into, the public channels they are in, private channels you are both in. There may be additional information depending upon what fields and modes they have set.
WHOIS is generally the first command you would use on people to learn something about them. For related commands see WHO, WHOWAS, and USERHOST.
Note: the UMODE +y for operators makes them see when someone performs a WHOIS on them. However, WHOISing is considered a valid tool for any user, so you shouldn't worry about it.