What the company tells you:
Companies are savvy corporate communicators. They recognize the value of having informative Web sites where they can showcase their products and services and communicate their vision. Many company websites have a history of the company and often of specific products. Often located at the bottom of the page, links to Investor Information, about us, social responsibility, etc. can lead to valuable information. Promotional materials, catalogs, product descriptions and price lists may also prove valuable. Remember that information provided by a company is self-reported and may be somewhat subjective.
What the company has to disclose to the government:
A variety of publically available reports are reported to the SEC (Security and Exchange Commission). The 10K is an annual report that provides a wealth of information about a company. Other SEC filings, the U.S. Census Bureau's Economic Census, state filings
What the company keeps confidential:
Trade secrets and other sensitive business information are often protected and kept confidential through non-disclosure agreements. Internal information is often kept confidential, as well.
What others say about the company:
A variety of information sources report what other say about a company. News, blogs, research articles, books, market research, analyst reports, government, industry and trade expert reports are available through both proprietary and openly available information sources.