Note that The Vanguard Total Stock Market Index Fund is not the same as the S&P 500 index. Your thread title mentions the S&P but the body of the post talks about VTSMX, so just wanted to make sure you were clear. The S&P 500 is only 500 stocks. Generally they are the large companies, so this is referred to as a Large Cap fund (Large Capitalization). The VTSMX has about 3300 3787 stocks, so you'll get small and mid cap exposure too. It's weighted by cap size, so most of your money still ends up in those same 500 stocks as the S&P 500, but they're not the same holdings.
Edit -- I just guessed on the number of different domestic stock holdings initially, then my curiosity got the better of me