I am sometimes asked the differences between Lean and Six Sigma. The “toolboxes” for operational improvement represented by these two approaches contain substantial overlap, particularly at the relatively basic level where most improvement initiatives start.
Lean seeks to maximise the value of a process to an end customer by elimination of waste in the process, waste being defined as anything that does not add value to the consumer, whereas Six Sigma seeks to achieve stability in a process by the elimination of variation through the process by the use of statistical improvement tools.
The overlap occurs because a wasteful process always suffers from variation.
It can also be argued that the Lean approach is a more macro approach that includes the management of human resources as very important to the improvement, whereas Six Sigma focuses on a more micro, quantitative approach to improvement.
The final irony in any discussion about lean, 6 sigma, and the TPS, is that it all comes from Henry Ford, who evolved a management system using the principals espoused in all three approaches, subsequently lost when he died, and his various writings ignored until the Japanese, post WW11 looking to rebuild their shattered economy came across them. If you did not click the hyperlink above, I suggest you do now for a brief history.