Major Stock indices, like the SP500, Dow Jones, or NASDAQ, focus on different Stocks. The S&P 500 is like an all-star game, the Dow Jones is a bit like the old-school team, and the NASDAQ is for tech lovers. As with other markets, and trading in general, which Stock Index you choose to follow and trade depends entirely on you. It is often useful to focus on markets, or in this case, Indices or Stocks that you are familiar with.