Before you begin researching your competitor company, you should know a little bit of information about these company. 
Why? It'll impact where you search and the amount of information that you find. Ask:
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 won't be included in some of the databases, such as NetAdvantage. Check Mergent or Business Source Complete for international company information.
Image Source: Ben Schumin. CC By-SA 2.5. Wikimedia Commons.
Company profiles provide succinct overviews of a company--company history, key facts, top executives, major products and services, competitors, locations, subsidiaries.
How do customers view the service or product they use now? Are there satisfaction studies on your competitors' products or services? Try searching for your product or service and satisfaction. Some databases may even have a subject heading for customer satisfaction.
Who are your competitors? Once you find them (if they are public companies) check out their 10K and other SEC filings for the company's annual report and more.
How will you differentiate your product or service? What would provide value to customers?
Access over 12,000 regional, national, and international full-text news sources.