VOO is a Vanguard ETF (exchange-traded fund) that tracks the S&P 500 and VFIAX is a Vanguard mutual fund that tracks the S&P. So, the difference is that one is a mutual fund (that trades at the end of the day) and one is an exchange-traded fund (that trades like a stock). There are more differences between ETFs and mutual funds, which you can read about here.

The relationship between VTI and VTSAX is the same. VTI is essentially the ETF version of VTSAX. Both are indexes that track the total stock market.

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