Message: an indirect procedure call.
A message is sent to an instance object.
The message contains a selector and optional
arguments. The selector is looked up in the class of the object (or one
of its superclasses) to find the method that implements
the message.
That method is called with the object to which the message was sent as
its first argument (called self, or this in Java).
Selector: the name of a message. A generic procedure name,
e.g. draw, that could apply to various kinds of objects.
Method: the procedure that implements a message.
Often the name of a method is the class name combined with the selector,
e.g. square_draw.