This command will give out the identifier on a user, which rooms the user is in, and the flags for that user. It will also show you the server he/she is using, the real name field information, the hostmask and flood credit.
Example:
/msg operserv userinfo Mark
This can be used if you need to see where a user is, or just want to get as much information as you can. But generally you will not need to check the userinfo on people.