Most active stocks are stocks with trading volume high enough to warrant inclusion on the most-active list. Some widely held stocks with many outstanding shares, such as AT&T and Intel, routinely make the list. Other stocks make the list because of special circumstances such as a tender offer or an unexpected news item about the issuer.

Each time you buy a stock, there's someone out there who thinks the stock is overvalued and is gladly selling his or her future interest in the stock for your cash now. In every transaction, someone is wrong.

In the most active stocks, this type of transaction happens millions of times each day. Much of that trading is done by large institutions, whose highly paid analysts make the buy and sell calls.