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:
SEC's EDGAR database provides free public access to company filings, allowing you to research a public company’s financial information and operations.
Private companies are not traded on any stock exchange and are not required to file detailed reports 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 an understanding that publicly available information beyond name and location might not exist.
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.