There's the standard S&P 500 index (SPX) and the rarer used S&P 500 Total Return index (SPTR). If you compare graphs, you'll find that the latter grows faster. Supposedly, SPTR assumes reinvestment of dividends while SPX doesn't.
What does SPX assume you do with your dividends? Put them under the pillow interest-free, or invest them at the risk-free rate? Even SPX must somehow account for dividends, or else each time a company issues a dividend (which is of course accompanied by a drop in stock price), the index would drop.
I assume (but do not know) that ETFs or index funds that claim to track S&P 500 would reinvest dividends, and so I'd expect that their prices would follow SPTR instead of SPX. However, from looking at graphs, the opposite seems to be the case. Obviously I'm missing something.