One of the greatest financial inventions over the last few decades has been the exchange-traded fund. Serving effectively the same purpose as a mutual fund but at a fraction of the cost, ETFs should be core holdings in virtually every individual investor's portfolio. 

With this in mind, Motley Fool contributor John Maxfield outlines the four things investors should look for when selecting an ETF, highlighting the Vanguard Total Stock Market ETF (NYSEMKT:VTI) and the SPDR S&P 500 (NYSEMKT:SPY) ETF, which tracks the S&P 500.

John Maxfield and The Motley Fool have no position in any stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.