A "method" is defined on an object, and contains behavior. In the examples, the Bottles class
defines a method named song.
A "message" is sent by an object to invoke behavior. In the above example, the song method sends
the verses message to the implicit receiver self.
Therefore, methods are defined and messages are sent.
Confusion about these comes about because it is common for the receiver of a message to define a
method whose name exactly corresponds to that message. Consider the example above. The song
methods sends the verses message to self, which results in an invocation of the verses
method. The fact that the message name and method name are identical may make it seem as if the
terms are synonymous.
They are not.
Think of objects as black boxes. Methods are defined within a black box. Messages are passed between them. There are many ways for an object to cheerfully respond to a message for which it does not define a matching method. While it is common for message names to map directly to method names, there isn't a requirement for this.
Drawing a distiction between messages and methods improves your OO mindset. It allows you to isolate the intention of the sender from the implementation in the receiver. OO promises that if you send the right message, the correct behavior will occur, regardless of the names of the methods that eventually get invoked.