type in objective-c

id:  a pointer to any type of object

BOOL::

YES: 1

NO: 0

IBOutlet:  a macro that evaluates to nothing. It is a hint for interface builder

IBAction:  the same as void,  a hint for interface builder

nil:  the same as NULL.  for pointers to objects

        In Objective-C, it is ok to send message to nil. The message is simply discarded.