The S&P 500 is a market-cap-weighted index of about 500 major U.S. companies, serving as a key benchmark for U.S. stock performance with an average historical return of around 10% annually before inflation. Investors can buy funds or ETFs replicating the index, which offer low expense ratios under 0.05%. Diversification across sectors reduces risk, and research shows lump-sum investing generally outperforms waiting for better entry points.