What’s the difference between __builtin__ and __builtins__?

Straight from the python documentation:
http://docs.python.org/reference/executionmodel.html

By default, when in the __main__ module, __builtins__ is the
built-in module __builtin__ (note: no ‘s’); when in any other
module, __builtins__ is an alias for the dictionary of the
__builtin__ module itself.

__builtins__ can be set to a user-created dictionary to create a
weak form of restricted execution.

CPython implementation detail: Users should not touch __builtins__; it is strictly an implementation detail. Users
wanting to override values in the builtins namespace should import
the __builtin__ (no ‘s’) module and modify its attributes
appropriately. The namespace for a module is automatically created the
first time a module is imported.

Note that in Python3, the module __builtin__ has been renamed to builtins to avoid some of this confusion.