A penny stock is a low-priced, speculative stock issue. Typically these companies have a small market capitalization, and a share value of less than one dollar.
"Pump and Dump" Internet schemes and email hoaxes or tend to focus on Penny Stocks, where the scammer focuses attention on a stock that they already own. They urge the readers to buy the stock, creating demand and increasing the price. The scammer can sell their stocks at a profit, and leave the other investors (both legitimate and those tricked) with a stock worth less than they paid for it.