If you are not familiar with penny stocks they are common stock and they usually trade for under $5 dollars a share even for pennies, hence the name penny stock.
They are traded over the counter also known as OTC through quotation services. The OTC Bulletin Board or pink sheets are quotation services.
Many people claim that penny stocks are thinly traded. But volumes traded daily can be in the hundreds of millions for a sub-penny stock.
True information about a penny stock company can be hard to find. And, penny stocks can be manipulated very easily by unscrupulous pump and dump operators.
If you want to trade penny stocks and avoid the pump and dump operators it is best to become a member of a reputable microcap/penny stock newsletter service.
Read here for Penny Stocks Pros and Cons