A public company has sold partial ownership to the public, usually through stock transactions. It is required to file details regarding its financial performance on a regular basis with the U.S. Securities & Exchange Commission (SEC).
The ownership of a private company is closely held. It has not sold partial ownership to the public and is not required to reveal the details of its financial performance through reports filed with the government.
It is usually much more difficult to obtain financial details about a private company. Look for news articles and evaluate the competitor information for public companies in the same industry.
See the Public or Private? page of the Business Research guide to identify resources.