Note the ticker symbol, NAICS and SIC codes. Publicly traded companies are required by law to disclose financial information to the Securities and Exchange Commission, and therefore public companies are much easier to research.
If the company is identified as being private:
Since privately held companies are not traded on stock exchanges, they are not required to divulge any financial information, hence finding extensive information on a private company can be difficult. Business publications are often the best source of information on a private company's activities.
Somewhat simpler to use, Mergent Intellect provides an overview, history, a description of products and operations, and a 'family tree' of company ownership.