Every message you type after that will go only to that person. If she then types
/query nickname
where nickname is yours, then you have established a private conversation. To exit this mode, type
/query
by itself. While in query mode, you and the other person can continue to "listen" to the discussion on whatever public channels you were on, although neither of you will be able to respond to any of the messages there.
/quit Exit IRC.
/signoff Exit IRC.
/summon Asks somebody connected to a host system with IRC to join you on IRC. You must use the person's entire e-mail address.
/summon fleepo@foo.bar.com
would send a message to fleepo asking him to start IRC. Usually not a good idea to just summon people unless you know they're already amenable to the idea; otherwise you may wind up annoying them no end. This command does not work on all sites.