What is a total stock market index fund and how is it different from an S&P 500 fund?

An S&P 500 fund holds about 500 of the largest U.S. companies, covering roughly 80% of the U.S. stock market's value. A total stock market index fund holds those same large companies plus thousands of mid-cap and small-cap firms, giving you essentially the entire U.S. market in one fund. In practice their returns track closely because large companies dominate both, but the total-market fund adds small and mid-size exposure that occasionally outperforms. Both are excellent, ultra-low-cost core holdings, and many investors reasonably pick either one. You don't need both, since they overlap heavily. If you want the simplest one-fund U.S. stock position, a total market index fund is a clean choice; an S&P 500 fund is nearly equivalent.