The term "Penny Stocks" most commonly refers to a common stock that is under five dollars per share and is traded over the counter (OTC). This is facilititated via quotation services such as the OTC Bulletin Board (OTC BB) or the Pink Sheets.
Penny stocks have garnered a reputation as being risky and volitile. While this is true, many great companies have started as a result of being listed as a microcap stock. Regardless, traders and investors should take great care when doing their due diligence as penny stocks are difficult to research and can be easily manipulated.
However, new regulations created have made trading penny stocks much safer. For example, the OTCBB now allows only securityies that completely report to be traded. Securities that do not report are often sent onto the smaller exchanges (such as the Pink Sheets). Also, recently some investors are referring to the OTC BB as the "Minor Leagues" - securities that are bounced from the NASDAQ often appear on the OTC BB before gaining ground and being relisted back on the major exchanges.
Here are some great resources for penny stock traders and investors: