Functions and methods are similar but not the same thing.
Methods provide a function-like behavior because they can be called and sometimes return a value. They operate on the content of the receiving object to which they are attached and yield information about the internals of the object.
Functions perform some transformation on the input arguments and return a result that is dependent on them. This computation is complete independent of the contents of any object.
Accessor methods to set and get internal values of an object are presented externally as properties.
A function is a construct that performs a task. It is derived from the procedural language model rather than the Object Oriented Language model. Procedural language functions are analogous to Object Oriented language methods but they are not quite the same. Functions are called and carry out some procedural task, while methods are invoked by sending a message to a receiving object - they may however belong to a class or an object instance of a class. Class methods are most likely to be factory methods for creating instance objects of that class.
Function calls can only be received by function objects otherwise a runtime error results - however, those Function objects generally belong to other objects and are stored as properties of the owner object.
A method is a specially written function that operates on the receiving object and can be shared between several objects using the 'this' variable to refer to the receiving object.
Associating a function with an object renders that function the active code to be called when a method is invoked on that object. Invoking a method sends a message to the object, the receiving object looks for a property with the message name which contains a reference to callable function object. If it doesn't have one, it calls its parent and delegates the message upwards.
ECMA 262 edition 2 - section - 11.2.3
ECMA 262 edition 2 - section - 11.2.4
ECMA 262 edition 2 - section - 13
ECMA 262 edition 3 - section - 11.2.3
ECMA 262 edition 3 - section - 11.2.4
ECMA 262 edition 3 - section - 13