Public companies are those that sell stock to the general public, and trade that stock on a stock exchange or over-the-counter market. The U.S. government requires publicly traded companies to file financial statements and annual 10-K reports to the Securities & Exchange Commission (SEC).
There are a number of filings the SEC requires but there are a few that are going to be the most helpful to those looking for what the company does:
U.S. private companies are not traded on any stock exchange and are not required to file detailed financials with the SEC (Securities and Exchange Commission). Thus, information is sparse and difficult to find. Most companies in the U.S. are privately held. Researching private companies requires out-of-the-box thinking and an understanding that it will likely be hard to find information beyond name and location.
When it comes to researching private companies, the first place to look would be the company's web page if they have one. Never underestimate the information that companies publish on their own web sites. Even if some companies use their web pages as glorified catalogs, brochures, or advertisements how they present that information may still be helpful. Often, for private companies, their web pages may be what provides the most information.
Nonprofit organizations with 501(c)(3) status are required to file Form 990, Return of Organizations Exempt from Income Tax, with the IRS as a condition of granting tax-free status. The form includes financial statements and background information on the nonprofit, including salaries of key employees (i.e. director, head, officers). Nonprofits are required to give a copy of their latest filing to anyone who asks. All 990 Form information is available from the nonprofit or the IRS. Most are available on the Web. To learn more about nonprofit tax filing visit the IRS site for Charities and Nonprofits. To understand how to read the 990, view this article from The Chronicles of Philanthropy entitled, "Mining the 990: A Guide to Gleaning Key Data From Charities’ Tax Forms."