Public companies sell stock to investors. They are required by law to release detailed financial and other information to the public.
Private companies do not sell stock. They often keep their finances private, and are more challenging to research.
Public companies usually post annual reports for shareholders under "Investor Relations" or "For Shareholders" on their website.
U.S. public companies must file accurate information with the Securities & Exchange Commission (SEC), which makes all filings publicly available on its website.
Non-U.S. public companies must file information in their home countries, but this information may not be available in English.