yield keyword usage

[Ehsan]

hi
coulde any one show me the usage of  "yield" keyword specially in this
example:


"""Fibonacci sequences using generators

This program is part of "Dive Into Python", a free Python book for
experienced programmers.  Visit http://diveintopython.org/ for the
latest version.
"""

__author__ = "Mark Pilgrim (mark at diveintopython.org)"
__version__ = "$Revision: 1.2 $"
__date__ = "$Date: 2004/05/05 21:57:19 $"
__copyright__ = "Copyright (c) 2004 Mark Pilgrim"
__license__ = "Python"

def fibonacci(max):
    a, b = 0, 1
    while a < max:
        yield a
        a, b = b, a+b

for n in fibonacci(1000):
    print n,