ETFs are reshaping retirement plans and 401(k)s. Learn how this $15 trillion sector could impact your nest egg and why more Americans are making the switch.

The S&P 500’s construction lends itself well to full replication: copying an index’s holdings and weightings exactly.

This measures the excess return of companies with high book-to-market value ratios – those trading at valuations that are systematically undervalued relative to their intrinsic worth. "The value ...