Public companies, which have shareholders and are traded on the stock exchange, are legally required to release specific financial information and other information about their business practices, making them much easier to research. You will often find much more information about public companies when conducting database research for companies.
Private companies are not traded publicly and therefore can keep much of the information about their financials and business practices private. Laws regarding private company disclosures vary from country to country, but private companies in the US are not required to release much information to the public. If you choose to research a private company, you may want to locate public competitors as a way of identifying information about the industry that would also apply to the private company.