// works as an "integer divide" in python3, take a look at this answer.
In C, division with / on integers works as a "division with floor" or "integer divide". In order to provide this capability, python provides the // operator, unlike / which will give a floating point result.
The authoritative reference is certainly pep-238.
From the command-line version (useful when you're trying to figure out things like this):
Python 3.2.3 (default, Apr 11 2012, ...
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 10
>>> a/3
3.3333333333333335
>>> a//3
3
>>>
