Stocks can trade two different ways, either Listed on an exchange or Over the Counter (OTC). When a stock trades above $1 and has a market capitalization of $400M or more, it is qualified to be listed on an exchange, like the NYSE or NASDAQ. If a stock is listed and falls below either of these requirements, or doesn’t meet the requirements, it may be delisted and begin to trade Over the Counter (OTC). This means that rather than trading on a listed exchange or centralized body that matches buyers and sellers, the stock is traded only by select wholesale dealers/market makers who trade the stock. 
These types of stocks have additional risks. Stocks that trade OTC have fewer reporting requirements, and fewer regulations. They also generally have fewer investors trying to buy or sell at any given time, and due to this limited demand, can be volatile. In some cases, the pricing you receive when you buy or sell a stock that trades OTC  may vary significantly from the last price it traded at it.

If a stock that was originally listed on an exchange falls below the $1 maintenance threshold and moves to the OTC markets, generally the stock ticker will change to a five letter symbol (ex: CRC > CRCQQ).

Also, to learn more about stock delistings, check out our learn article: https://learn.stash.com/what-happens-when-a-company-gets-delisted