Both of them are well-recognized process improvement tools/concept, but with very different nature and focus;
lean--focusing on speed of process & waste reduction;
six sigma--focusing on variation reduction

谢谢！

Basically, we use Lean to remove sources of defects and wast we can "see", while we use Six Sigma to remove sources of defects and waste we cannot "see".
And the common goal of the both is to speed up and smooth our process, for cost-down and profit-margin purpose.