A stock index is a method of tracking the overall movements of the stock market, or a particular market sector or other subset of the market. Common indexes include the Dow Jones Industrial Average, a basket of 30 Large US company’s stock, the S&P 500 Index, an index of 500 large US stocks, and the NASDAQ-100, an index of 100 companies whose stock trades on the NASDAQ stock exchange. The movements of a stock index are calculated by taking the individual movements of each component stock and then applying them in aggregate to the index itself. Exactly how this is calculated varies among the indexes based on whether they are value-weighted, size-weighted, or other methodology.
The indexes themselves are simply mathematical expressions of the movements of individual components. Therefore, investors cannot invest directly in an index since no actual index security exists. The closest thing to investing in an index is to purchase a security which tracks the index the trader is interested in.