The Dow is named after Charles and his associate Ed Jones. The Dow average is a price-weighted index and the value of the Dow is not the actual average of the prices of its tracked stocks, but rather the total of the stock prices divided by a divisor, which changes whenever one of the component stocks has a split or dividend. The Dow probably the most popular benchmark for tracking overall US stock market activity & exchange health. All quotes here are delayed.
The S&P also is short for the Standard & Poor's 500, a US stock index based on the market cap of 500 companies. The S&P is different from other indexes because of its diverse constituency and weighting methodology. The S&P is considered another good metric to summarize the overall health of the US economy.
The Nasdaq is the world's first electronic stock market. NASDAQ is the largest electronic stock market with over 3,000 companies listed.