__builtin__ vs __builtins__?

[rihad]

Hi there, I can't seem to understand the difference between
__builtin__ and __builtins__. Both are builtin (yuck) modules, the
contents given by dir() look the same, the only difference I was able
to spot was that __builtins__ is imported by default (dir() with no
args shows it), whereas __builtin__ has to be imported by hand.

(BTW it looks like all the 115 items can be reference without
qualification)

Which should be used?