All publicly traded companies in the United States are required to file an annual statement, called a 10-k, with the US Securities and Exchange Commission. (Publicly traded companies in other countries have reporting requirements and governmental agencies of their own, which are not covered here.) The 10-k is a comprehensive, detailed report of a publicly traded company's business operations and financial condition. It's usually accompanied by additional statements: executive compensation, financial statements, and so forth. While publicly traded companies also file additional reports with the SEC (10-Qs or quarterly reports, for example), the 10-k is the most important for researchers.
Corporations based outside of the USA but selling shares on any US stock exchange are required to file a 20-f annual statement with the SEC. Similar in scope to the 10-k, its purpose is to standardize reporting for international companies.
By law, both the 10-k and the 20-f must be made available to shareholders. Companies usually comply with this requirement by posting the statements to their websites. You can also find 10-k and 20-f statements by searching the SEC's website search engine, EDGAR.
To learn as much about a company as possible, you should read the entire 10-k or 20-f, of course, including the attached supplemental documents. However, if you just need some background about the company in question, you can limit yourself to reading the following: