CTCP

Client To Client Protocol (CTCP) is perhaps a missnoma, as CTCP requests are still sent via the IRC server. They allow a variety of interaction with other clients beside basic chat functionality, such as:

CTCP Command	Effect
/ctcp nick ping	Requests a ping from that client. useful for knowing if they're lagging.
/ctcp nick time	Requests the time from the client. useful for telling of two clients are on the same computer.
/ctcp nick version	Requests the client version

A user can edit their client's response to CTCPs, so don't believe everything you read.