Indices (S&P, Nasdaq, Dow)

An index = a basket score

An index tracks a curated basket of stocks: the S&P 500 (500 biggest US companies), Nasdaq 100 (tech-heavy), Dow Jones (30 industrials). You can't buy 'the index' directly, but you can trade ETFs (SPY, QQQ, DIA) or index futures (ES, NQ, YM) that track them.