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.