In the US, a public company: (a) offers stock for sale publicly on a stock exchange, and (b) makes public certain business/financial information as dictated by the Securities & Exchange Commission (SEC).
Private company shares are held by a small group of investors and are not publicly traded. Private company are NOT regulated by the SEC, so they are not compelled to disclose the same information.
Private company research is trickier than public company research.
Adapted from Introduction to the Markets: Public Companies from the U.S. Securities and Exchange Commission.
These online databases contain profiles on private and public companies. The private company information is limited but may include sales and employment estimates or reported data.
Comb the company website for information. Here are some tips: