The majority companies are classified as either public or private. Public companies sell stock, therefore anyone from the general public can buy shares and become an owner of the company. In addition, public comanpies are required by U.S. Securities and Exchange Commission to release information about their finances and operations.

Private companies do not sell stock to the general public. Stock is usually owned by the principles in the company, and as a result, they are not required to submit financial data to the SEC. These companies do not publish annual reports or share information about themselves to major business publishers. Therefore, information about private companies is generally difficult to find.

Resources for Private Companies

Hoover's Handbook of Private Companies 
Lewis Ready Reference HG4009 .H668 
Profiles major U.S. private companies

International directory of company histories 
Lewis Reference HD2721 .I48
A multiple volume (& still growing) reference containing information about the "historical development of the world's most important companies." Use the index (alphabetical, industry, & geographic) in the most recent volume.

Ward's business directory of U.S. private and public companies 
Lewis Ready Reference HG4057 .A575
Over 114,000 companies arranged alphabetically, by geographic location, and by industry classification (NAICS & SIC). The company rankings by sales within SIC and NAICS classifications are very useful.