To the best of my knowledge, .groups() returns a tuple of remembered groups. I.e. those groups in the regular expression that are enclosed in parentheses. So if you were to write:
print re.search(r'(1)', '1').groups()
you would get
('1',)
as your response. In general, .groups() will return a tuple of all the groups of objects in the regular expression that are enclosed within parentheses.