What is an Exchange?
An exchange is an institution, organization, or association which hosts a market where stocks, bonds, options and futures, and commodities are traded. Buyers and sellers come together to trade during specific hours on business days. Exchanges impose rules and regulations on the firms and brokers that are involved with them. If a particular company is traded on an exchange, it is referred to as "listed".
Companies that are not listed on a stock exchange are sold OTC (short for Over-The-Counter). Companies that have shares traded OTC are usually smaller and riskier because they do not meet the requirements to be listed on a stock exchange.