Before you begin company research, you should know a little bit of information about your company.
Why? It'll impact where you search and the amount of information that you find. Ask:
Image Source: Ben Schumin. CC By-SA 2.5. Wikimedia Commons.
Private companies are more difficult to research because they are privately owned; they do not have to disclose any information to the public.
Public companies are easiest to research. They are regulated by the government (in the U.S., it's the SEC), sell shares to the general public, and must publically report company and financial information.
Subsidiary companies will not have their own annual reports, and you may not find information on them in the resources listed on this page. You'll need to determine who the parent company is and research the parent company. Journal articles will be a major source for information on subsidiaries.
International companies also may not be included in databases, such as Business Source Complete.
Company profiles provide succinct overviews of a company-- including company history, key facts, top executives, major products and services, competitors, and locations/subsidiaries.
"Publicly traded companies that meet certain criteria are required to file financial reports with the Securities and Exchange Commission (SEC). 
One of those reports is the 10-K, a document filed annually that is sometimes referred to as a company's annual report.
Because the 10-K includes a tremendous amount of detail and legal jargon, most companies also produce a more reader-friendly annual report that typically contains product information, photographs, charts, a letter from the CEO, and other company and financial data." Source: IRIN
Image Source: Design by Erik Möller (Eloquence), photo by Everlong. CC-BY-SA-3.0. Wikimedia Commons.
Annual Report Service (WSJ) Free service for investors seeking stock research & financial information."
Public Register's Annual Report Service (PRARS) Order reports from participating public companies. Reports delivered free of charge within 24 hours.
The sources above provide free annual reports; registration may be required.
A 10-K, which is a report filed by publically owned companies with the Securities and Exchange Commission (SEC), provides information such as company history, organizational structure, executive compensation, equity, subsidiaries, and audited financial statements.
Following are resources that'll take you to 10-K's.