They both track the same stock market, but sometimes you wouldn’t know it.
The S&P 500 and the Dow Jones Industrial Averages are the two most-quoted indexes of stock market activity. Yet they often provide very different impressions of how the market is performing. Why? The short answer is, they each track a different basket of stocks, with different weightings, and are influenced by the human beings that make the selection. The S&P includes many more companies, including smaller-cap companies that tend to gain more, percentage-wise, in market upticks, while the smaller set of Dow companies is skewed towards more stable, large-cap firms.
Generally speaking, in upward-moving markets, the S&P tends to report more positive performance, while the Dow holds steadier in declining markets.
For more details on these differences — and how they affect the calculations of the indexes, and how that affects how each should be interpreted — read this excellent article from Marketwatch.