Yield Keyword

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A function where the yield keyword is used, is a generator.
Calling the function returns a generator object. This objects saves it's internal state.
The generator object is also an iterator.
This is where 'magic' comes in-place. Calling next() on a generator
object, executes the code until the first yield has been reached.

def gen1():
    yield 1337

g = gen1() # no code execution
next(g) # gives you 1337
next(g) # Raises StopIteration

Since Python 3.4 you can and should use the return statement inside a generator to stop the iteration. It's better then raise StopIteration, which makes trouble with newer version of Python.

The Fibonacci example has a loop, where the yield keyword is hit more than one time. This is the cause, why it behaves like a loop.