CLIENTINFO returns the known CTCP commands from another client in a list. You can inspect commands further by calling 'CLIENTINFO VERSION' for example. You will (or should) be given a one-line explanation of what this command is supposed to do.
To find out about your own client services, execute a CLIENTINFO on yourself: CTCP mynick CLIENTINFO
Warning: Some CTCP commands are not supposed to be sent by hand, in particular ERRMSG, DCC, and SED.