This debate has been going on for years. The isixsigma link provided by Jennifer is a good example. Also, because this is a SS forum, there is more pro SS than lean. I have spent a few months on this forum and found some excellent examples of SS tool applications as well as anal-retentiveness. The other link sent by Jennifer is a little better, but from my prespective, also a little one sided to SS thinking.
In a nut shell, SS works at reducing process variation. So, any process that is out of control needs first to be put in control before SS can work. This can be done with lean.
I believe the author of this other document link provided by Jennifer did not clearly understand that lean also meticulously uses strong statistical process control. As with SS, the target is a minimum of 1.33 Cpk. Where SS takes over is to improve the process capability even further.
So, what is the real difference.... from my perspective ... Six sigma ppl perfer to use SS principles, while Lean ppl perfers to use lean principles and 25% along the way, they both intersect in the usage of the same tools.