Nettuts+ have an excellent article about Javascript object oriented programming and they came up with a very clear definition to the keyword 'this' as below:

So what does ‘this’ reference? Well in a browser, ‘this’ references the window object, so technically
the window is our global object. If we’re inside an object, ‘this’ will refer to the object itself however
if you’re inside a function, this will still refer to the window object and likewise if you’re inside a method
that is within an object, ‘this’ will refer to the object.

>Due to our scope chain, if we’re inside a sub-object (an object inside an object), ‘this’ will refer to
the sub-object and not the parent object.

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