Public Companies: Note the ticker symbol, stock exchange, and the NAICS & SIC codes. Publicly traded companies are required by law to disclose financial information to the Securities and Exchange Commission (SEC), and therefore public companies are much easier to research.
Private Companies: Since privately held companies are not traded, they are not required to release any financial information. Therefore, finding information on a private company can be difficult. Articles are often the best source of information on a private company's activities.
Subsidiaries & Affiliates: A company's subsidiaries are not analyzed individually. Try searching [i.e. 'find in page'] within the parent company's annual reports, and business news and journals, for information on the activities of a subsidiary.
All of the Hoover's Handbooks include brief snapshots of companies, with descriptive histories.