Six Sigma is a data-driven methodology for process improvement that aims to reduce defects and errors to a very low level (3.4 defects per million opportunities). It follows a structured approach called DMAIC (Define, Measure, Analyze, Improve, Control) for existing processes and DMADV (Define, Measure, Analyze, Design, Verify) for new processes or products, using statistical tools and techniques to achieve high-quality results.